Abstract: Since 1932 when Sherman put forth a unit hydrograph concept, Sherman unit hydrograph has been a basic hydrological method for solution of complex runoff concentration. The authors analyze the current methods of unit hydrograph calculation and their problems, and present a stepwise filter optimization method. This paper gives objective function and tconstraint conditions for this method, elaborates the theory of moving average filter and moving polynomial filter, and demonstrates a procedure for its calculation of unit hydrograph. Its application to calculation of Sherman unit hydrographs for different river basins has achieved satisfactory results.