The filter in micro-irrigation is the key to guarantee normal running of the whole micro-irrigation system. It is the key to develop automatic filter. Based on the research of hydraulic performance of drain system for automatic suction screen filter, the discipline of filtering time for automatic suction screen filter was analyzed. It showed that the change of filtering time was not linear. The factors affecting sewage time were analyzed. The results showed that the cleaning-time was not the constant value, but had the inverse relation with inflow sediment concentration S0 when other parameters were constant.