Abstract: Flow rate of inner wheel irrigation ditch is generally determined according to irrigation rate and irrigation area.In case of different irrigation areas controlled by head ditch in irrigated areas，net flow of head ditch and field ditch should be distributed according to proportion of irrigation area of each channel in irrigated area of irrigation group.It is simple and accurate to calculate discharge of each canal by calculating average number of agricultural canals controlled by each ditches，and it can be used as a method to determine scale of ditches by checking calculation and practical test by relevant standard formula.