Abstract: Abstract: Straw mulching is a main method to reduce soil erosion and improve soil moisture. However, overmuch straw mulching may lower the anti-blocking capacity of no-till planter, affecting seeding quality and grain production in conservation tillage system in North China Plain. So, more researches should be conducted to investigate the suitable straw mulching quantity in conservation tillage system. Aiming at achieving suitable straw mulching conditions for high-performance planting of no-till planter and efficient soil and water conservation, this paper studied the effects of straw mulching on slope runoff and sediment yield process under different simulated rainfalls, and quantified the straw mulching rate for different conditions. A rainfall simulation experiment was carried out at Zhuozhou Experimental Station of China Agricultural University, Hebei province, from October 2013 to May 2014. The soil-bin (length: 1m, width: 1 m, slope rate: 9%) was filled with sandy loam soils according to natural soil structure in North China Plain. The surface was covered by chopped (less than 100 mm) maize straws collected after maize harvesting (early October). A rainfall simulator (model: DIK-6000, made in Japan) was used to produce different rainfall intensity. In this experiment, eight kinds of rainfall intensity (10, 20, 30, 40, 50, 60, 70, and 80 mm/h) and five kinds of straw mulching rate (0, 15%, 30%, 60%, and 90%) were considered as two independent variables in experimental design (three replications). Each simulated rainfall lasted for 1 h. The results showed that under different straw mulching rates, runoff time and rainfall intensity varied as power functions in conservation tillage fields in North China Plain. Straw mulching significantly (P<0.05) delayed runoff time in each simulation, and compared with uncovered treatments, straw mulching treatments with a coverage of 15%, 30%, 60% and 90% delayed runoff time by 1.0-15.4, 2.1-22.1, 3.4-48.2, and 5.9-73.6 min, respectively. Furthermore, straw mulching restrained runoff and reduced total runoff and total sediment yield. Compared with no straw mulching treatment, 30% straw mulching treatment significantly reduced total runoff by 17.9%-38.7%, but the positive effect in 15% straw mulching treatment was negligent. For sediment yield, the value in 30% straw mulching treatment was 34.1%-48.0% lower than that in no straw mulching treatment under total 8 kinds of simulated rainfall conditions. However, significant reduction (15.6%-34.7%) in 15% straw mulching treatment could only be founded in heavy rainfall (50-80 mm/h) experiments. The results above demonstrated that >30% straw mulching rate could significantly reduce soil erosion and improve soil water retention capacity. However, in North China Plain, the annual output of maize straw is about 7000 kg/hm2. In conservation tillage fields, if the straw mulching rate is over 60%, and this heavy straw cover condition will affect no-till planter’s anti-blocking capacity. So, in conservation tillage system, 30%-60% maize straw mulching (1.4-3.1 t/hm2) is recommended to achieve high performance of planting and high efficient soil and water conservation, which provides the valuble information for suitable straw mulching rate of maize in North China Plain during the application of conservation tillage practice.