Abstract: Abstract: The optimal design of a sprinkler irrigation system belongs to a class of large combinational optimization problems. It involves choosing the proper type and number of sprinklers and pipe diameters, and operating them at rational pressure heads according to the characteristics of pump and motor equipment. Theoretically, the optimization has rarely been addressed on the small-scale sprinkler irrigation systems compared to applications of the systems. Based on a survey done in cooperation with the factory, a small-scale sprinkler machine type 4.4CP-45 that can be equipped with fluidic sprinklers 10PXH, impact sprinklers 15PY, 20PY or 40PY, was taken as an example to investigate the impact of designs on the performances of system laid on a flat ground so that the characteristics of different designs involved can be summarized. The optimization models were built with three objective functions considering the resource consumptions per unit area irrigated, specific energy consumption, annual cost and total cost in a life cycle of the system separately subject to the constraints considering the pump-pipeline working conditions. The specific energy consumption here refers to the energy consumption per unit area with unit depth of water applied, and a static depreciation method was applied in the annual cost, while a dynamic method in the total cost. In the hydraulic calculation a back step method was introduced, and the optimization models were solved with genetic algorithms. Results show that: comparison of optimal designs under three objective functions will reflect the advantages and disadvantages of every scheme involved, which is in accordance with field practices and applications. When the irrigation system is equipped with one sprinkler typed 40PY, the specific energy consumption is high, and annual cost very low, so it's suitable for the irrigation of lands in serious drought or for the field crops. When equipped with 15 sprinklers typed 15PY, the specific energy consumption of the system under optimal design is 7.3% lower than that before optimization, 8.3% lower than that of the system equipped with sprinklers typed 20PY. All the indicators of the system are low, and it will show a high irrigation uniformity, but it is labor intensive in moving, thus it's applicable for profit crops or young plants. When 7 sprinklers typed 20PY are used, the total cost of the system is at its lowest, 6 284.8 Yuan/hm2, 15.4% lower than that of the initial design, and it is practical for many occasions. In the economical analyses of the system, the energy consumption fee or operation fee in any scheme constitutes a major part of the annual cost or the total cost respectively. Therefore, to reduce the energy consumption of irrigation systems through optimization or introduction of low pressure sprinklers is now an important task.