The polyurethane/polsytrene interpentrating polymer net (PU/PS IPN) was perpaed by simultaneous polmerization method. The effect of preparation conditions on the reaction rates of each component were investigated. The reaction rate of PU component increased with the increase of concentration of crosslinking catalyst(OcSn) of PU component and molecular weight of PU prepolymer. The reaction rate of PS component increased remarkably with the increase of initiator(BPO) concentration, crosslinking agent(Trim) of PS component and with the decrease of molecular weight of PU prepolymer.