Abstract: The graphic method for dynamic emptying storage coefficient was an irrigation optimal allocation based on function of the least waste water level of reservoir system. This method mentioned above was applied to irrigation optimal allocation for small reservoir system. By using this method, the emptying storage coefficient of a reservoir at any time indicated the capactity of water storage and water supply of the reservoir at that time. The water storage, water drawing off order and capacity of a reservoir system were determined by the emptying storage coefficients of every reservoir at the initial time. The emptying storage coefficients of every reservoir were kept equal at the end of the interval under constraints of reservoir system. In this way, runoff was retained in reserviors as much as possible and the waste water level of reservoirs was the lowest.