Abstract: In order to imitate accurately flow deviation in drip irrigation system, based on the hydraulic theory, the variation regulation of the headloss of the lateral under different slopes was analyzed, and the dripper distribution of maximal and minimal working pressure under different slopes were determined. Taking into account of hydraulic variation and terrain slope, a formula to compute the flow deviation of single lateral and the method for calculating flow deviation in blocks were deduced. Compared with calculation method in technical standard of micro-irrigation projects, calculation result is more consistent with actual situation. It can provide the more accurate technological guidance for designers, and makes that the practical operational targets keep pace with the designed target in drip irrigation system.