In order to overcome the weaknesses in the traditional methods for base flow separation, such as the lack of both physical basis and feasible proeedures, three basic principles for base flow hydrograph separation were proposed,based on which a new base flow hye]rograph separation method was developed. In this new method,the mathematical expressions of three parameters for Horton infiltration capacity curve were derived in terms of rainfall and runoff data, and the lag time of the effective rainfall in unsaturated zone and the groundwater flow routing equation were also presented. With these equations,the surface runoff and base flow hydrographs at the outlet of the basin can be scientifically separated.