Abstract: Abstract: Discharge time is an important functional parameter of filter. Discharge time affects the normal operation of micro-irrigation system and the irrigation quality of field crops. If the discharge time is too long, it will lead to waste of water source and influence the uniformity of drip water. If it is too short, the filter will not be able to discharge effectively. Therefore, it is very important to determine the optimal and practical discharge time of the filter in micro-irrigation system. Through repeated experiments and theoretical calculation methods, experimental study was carried out on discharge time of torpedo screen filter (screen mesh were 80 and 120). Under the test condition of 0.04, 0.05, 0.06, 0.1 MPa of set differential pressures, two groups with different influent sediment content were used to carry out the test. The results show that, the variation of sediment content with discharge time is basically the same under the conditions as influent flow of 300 m3/h, different preset pressure difference and different sediment content condition. The initial drainage sediment concentration and peak value of drainage sediment concentration occur while influent sediment content was high. For 80 mesh and 120 mesh filter, the drainage time of reaching to the stable sediment content is approximately same, which is mainly related to the thickness of the sediment particles used in the test, the adhesive strength of the cake formed on the inner surface of the filter net, the time required for the discharge of the coarse sediment particles from the torpedo parts, influent sediment content and the sediment content in the sewage discharge. The peak time of sediment content is about 10 s as the discharge time continues, when the discharge time is close to 10 seconds, a thick stream of sewage will flow out from the end of the sewage pipe, which shows that the thicker silt cake on the inner surface of the filter screen is carried into the sewage pipe by high pressure and high speed flow. The inflection point of discharge curve occurs when the discharge time reaches 15 s, the sediment on the inner surface of the filter screen is getting smaller, so the sediment content of the sewage decreases sharply. The sediment content of sewage is basically stable within 40-50 s. Through theoretical analysis, discharge time calculation method of the torpedo screen filter was derived. The method for calculating the discharge time of torpedo filter is an expression based on the traditional self-cleaning filter, considering the sand removal function of torpedo parts, it is necessary to adjust it properly. The theoretical discharge time calculated by this method is in the range of 40-70 s (for 80 mesh filter) and 20-70 s (for 120 mesh filter), which is basically close to the optimum discharge time of 40-50 s obtained by the test. The theoretical discharge time is close to the discharge time 40-50 s, which is obtained by experiment. Combined with experimental research and theoretical analysis, it is determined that the optimum discharge time of torpedo screen filter is 40-50 s.