Runoff forecastin the semi-arid and semi-humid region is characterized by the excess holding and excess runoff. The calculation of subsurface flow is the key to solve the problem because itis directly associated with simulation accuracy. Based on the analysis of the“double excess”model,the Xinanjiang model,and so on,this study finds thatsubsurface flow maybe occur in unsaturated soil and is induced by spatial difference of runoff generation. By referring to the advantages of subsurface flow model,a calculation modle for subsurface flow in the semi-arid and semi-humid region is developed using the thoughtof system model. The accuracy of developed calculation model for subsurface flow is then improved by measured dada. The computational method is notonly applied to the“double excess”model,butalso other similar models.