Rigid polyurethane foam (RPUF) with different densities is suitable for different engineering applications. The work aims to study the mechanical properties of RPUF with different densities and modify the existing constitutive model to obtain the real constitutive relation of the material. The stress-strain curves of RPUF materials with different densities were obtained by conducting quasi-static compression tests (0.001 s−1) and dynamic compression mechanical properties tests at different loading rates (100-1 000 s−1). On this basis, the density effect and strain rate sensitivity of RPUF materials were analyzed to obtain the real constitutive relation. The mechanical properties of RPUF materials affected by the coupling effect of strain rate effect and density effect, and with the increasing density and strain rate, the yield stress of RPUF materials increased exponentially, giving the functional relation between yield stress, strain rate and density of RPUF materials. A modified Sherwood-Frost model (M-S-F model) based on experimental data and lateral inertia effect is obtained. The research results can provide guidance for the engineering application of RPUF materials in impact resistance.