To study the change rule of mesoscopic interface performance of HTPB/IPDI (hydroxyl-terminated polybutadiene/isophorone diisocyanate) composite solid propellant with the loading rate, based on the molecular dynamics algorithm, the mesoscopic particles packing model of HTPB/IPDI composite solid propellant was generated. Bonding effect between particles and binder was modeled by a rate-dependent cohesive zone model constructed via combining viscoelastic standard mechanical units and exponential type rate-independent cohesion zone model(CZM). The relaxation parameters of the matrix material in the meso-scle finite element model were obtained through stress-relaxation tests of HTPB/IPDI curing films. The macroscopic mechanical response of HTPB/IPDI propellant at different loading rates of 0.1, 5 mm·min^-1 and 20 mm·min^-1 was simulated and calculated based on the model. The inversion analysis of rate-dependent cohesive zone model parameters was performed through Hooke-Jeeves optimization algorithm using numerical simulation results and the uniaxial tensile test results curve of of HTPB/IPDI propellant. The optimized values of interface parameters were obtained. The macroscopic mechanical behavior of HTPB/IPDI composite solid propellant at the loading rates of 50 mm·min^-1 and 100 mm·min^-1 was predicted using the established model. Results show that the predicted results are consistent with the actual experimental ones.