Polymer Bonded Explosive （PBX） is a multiphase composite material composed of pure explosive particles occupying a high volume fraction and a little polymer binder. The interface debonding between particles and binder and the mesostructure play a critical role in the mechanical properties of the material. In this paper， according to the stochastic simulation method combined with the Voronoi method， the representative volume element model of PBXs is established at the mesoscale. When the particles generated by Voronoi method are multi-graded， the particles around the large ones show a strip-scattering shape. The method of mesostructure modeling of PBXs was improved based on Voronoi method. Considering the mesoscopic interface characteristics of PBX-9501， the interface damage evolution between particle and binder under static tension was numerically simulated by using the constitutive relationship of three-stage bonding interface. The results show that the macroscopic mechanical properties of PBX-9501 agree well with the experimental data. The relationship between convergence and the size of representative volume element is discussed. It is concluded that the larger the size of the representative volume element is， the worse the convergence of the interface debonding simulation is.