The precipitation within a return current process over Northern China in May 24—25 1991 is analysed. It is considered that:(1) The structure of the return current process precipition, NE cold flow at low level and SW warm and wet flow above, form the warm front precipitation cloud system in favour of the weather modification cloud seeding; (2) The precipitation enhancement is closely related with the cold air action in the boundary layer; (3) The deep inversion layer is actually the transitional level of NE and SW wind. The vertical wind shear above and below the inversion layer is very strong. So that the Richardson number is less than 1.0 within all the layers,and it is a mechanism of precipitation enhancement.