The polyurethane/polystyrene interpenetrating polymer network (PU/Ps IPN) was prepared by simultaneous polymerization method. The effects of preparation conditions on the reaction rates of each component, domain size, effective crosslinking density, Tg, mechanical properties were investigated by SAXS, DSC and mechanical analysis. The increase of reaction rate of PU component caused the decrease of Tg of PS component even in the constant composition on PU/PS IPN. This result is considered that the increase of reaction rate of one component changes the matrix phase in IPN to its own phase. The notched impact strength of PU/PS IPN increased with the increase of reaction rate and molecular weight of PU component, but decreased with the increase of reaction rate and crosslinking density of PS component.