In order to quantitatively analyze the variation of rainfall infiltration recharge resources obtained by groundwater under the condition of changing underlying surface and the relationship between surface water and groundwater conversion in mountainous-plain areas, the long time series SWAT runoff model and the groundwater recharge from mountainous area to plain area in Sumihuai area of Beijing were selected as transfer variables to replace rainfall infiltration recharge and lateral recharge in the piedmont region in MODFLOW model, respectively. The simi-loose coupling SWAT-MODFLOW was established. Through verification, we found that the coupling model can accurately depict the change of groundwater level in this area, and the method of transferring rainfall infiltration recharge to MDFLOW model can reflect the recharge ability of groundwater in SWAT model. The rainfall infiltration recharge calculated by the coupling model is 1.31×108 m3/a , which is 0.54×108 m3/a less than that calculated by traditional rainfall infiltration coefficient method. It reflects that when the underlying surface conditions change and the thickness of vadose zone increases, the rainfall recharge of underground aquifer decreases.