Abstract: Abstract: Water resource is distributed unevenly in time and space in the world. With the development of society, water demand is increasing. In some places, water shortage is becoming the bottleneck of the development of social economy. In order to realize the rational allocation of water resources, lots of water diversion projects with cascade pumping stations have been carried out. Water is transferred from areas with plenty of water to areas with more water demand. For example, South-to-North Water Transfer Project is being built in China, which is the largest water transfer project in the world. There are 3 lines for this project, which are East line, Middle line and West line. East line is typical parallel and series cascade pump system, which is the largest pumping station system in the world. Water is pumped from Yangzi River in Yangzhou of Jiangsu Province to North areas including Jiangsu and Shandong Province. At some places old rivers are used for water transfer such as the Grand Canal between Beijing and Hangzhou. At other places new channels are built for water transfer. Some lakes used for storage are connected by these channels and rivers such as Hongze Lake, Nansi Lake and Dongping Lake. From Dongping Lake water flow becomes gravity flow to 2 directions. One route is north to Tianjin finally and the other is east to Jiaodong area. Water transferred to Jiaodong belongs to the first stage. Now the first stage of East line has been finished and is on operation period. It is typical cascade pumping station system for water transfer project. Large scale pumping station in parallel and series is a complex water transfer system. It consists of water pumps, controlling gates, channels or pipes and water lakes or reservoirs, and so on. Water is pumped from low area to high position or pressure pipes. Then it is transferred by channels or pressure pipes to water lakes or reservoirs. Lakes and reservoirs are used for storage and volume adjusting. In this system, pumping stations at different levels are control units. Channels or pipes between pumping station units limit the adjusting ability on water quantity. What's more, discharge and water level interrelate. During the operation period, water flow is changing dynamically caused by different scenarios or unknown external disturbance. So this system has the characteristics of complexity and dynamic. It is difficult to evaluate and optimize the real-time operation state. So how to control the system accurately becomes a challenge. Normally operation efficiency is one of the important methods for checking the validity. The operation efficiency of cascade pumping stations for water transfer system reflects the operating results of the whole system. It determines the energy consumption and the cost of water transfer. So it is an important evaluation index of system operation. It can also be one of the criteria to measure the success of water transfer project. There are many methods which can be used to calculate out operation efficiency. But these methods are focusing on end efficiency or average efficiency, which cann't reflect the accurate system running status in real time or a time interval. In order to solve these problems, with a series of research or experiments, the operation efficiency theory based on time and space is presented. In space scale, this system is divided into several subsystems, which are parallel subsystem, series subsystem, pumping station subsystem and water transfer subsystem. Pumping station subsystem consists of different pumping stations at different levels, which is the center of energy transfer of the whole system. Water transfer subsystem consists of different hydraulic structures such as channels, pipes, trash rack, and so on, which is the link of energy transfer. Different subsystems are connected by hydraulic factors such as water level or discharge. The operation efficiency of the whole system is decided by different subsystem. In time scale, it is divided into several parallel states, which are used to calculate pumping station performance, hydraulics loss and water quantity loss. A series of index system and formulas for operation efficiency of large scale pumping station are given out including pumping station subsystem and water transfer subsystem. Based on this, the related influence factors are studied with quantitative analysis. This research provides the theoretical basis for optimizing the operation efficiency of large scale pumping station system. Based on real operation data in 2016 of typical water transfer system with cascade pumping station in parallel, practice is carried out for this theory. Results show that this method can evaluate the operational efficiency accurately and find out the optimized potential factors and bottlenecks of the subsystems and the whole system. The operation efficiency of cascade pumping station system based on time and space scale is decided by different related factors such as water level, discharge and pumping unit power, and so on. So it is necessary to monitor these factors accurately. At the same time, summaries should be made in time about pumping station unit performance, hydraulics, discharge loss, and so on to find out the useful law. In one word, accurate operation efficiency comes from accurate raw data and detailed study. In addition, the operation efficiency can also provide the reference for route comparison and model selection of all-level pumping stations, and therefore, it can also be used for project planning.