The micromechanical damage of a composite solid propellant was observed by insitu scanning electron microscopy. Based on the damage characteristics, a cohesive interfacial element was adopted to model the debonding processes along the particles and the binder interface. The effects of interfacial strength and microcracks in the binder on the debonding process of propellant were also examined. The results show that interfacial debonding is the propellant's main failure mode under tension. Finite element method analyses with a cohesive interfacial element could predict the heterogeneous strain and stress fields as well as the processes of the particles separated from the binder. Interfacial strength plays a significant role in macroscopic behaviors of the propellant. Microcracks in the binder significantly influence the debonding process. The numerical simulation results reasonably reflect the corresponding experimental results. These results provide the basis for the prediction of mechanical properties and the optimal design of the composite solid propellant.