To estimate more accurately the relaxation modulus of composite solid propellant with different particles volume fraction, the molecular dynamics method was adopted to construct solid propellant meso-scale models with different particle volume fraction. The mean stress of composite solid propellant meso-scale model varying with relaxation time was computed by finite element theory and mesomechanics homogenization method when the model was under a fixed strain load. So the effective relaxation modulus of composite solid propellant can be estimated. It can be seen clearly that the instantaneous modulus of composite solid propellant increases when particle volume fraction becomes larger and the influence of particles random distribution to the instantaneous modulus of composite solid propellant by the proposed method.When this method is applied to the design process of composite solid propellant, it can reduce the design cost and shorten the design cycle effectively.