The effect of particle/matrix interface debonding on mechanical behavior of HTPB propellant was studied.The three-phase model consists of particles, vacuoles and matrix were given.The constitutive of HTPB propellant was obtained by the algorithms.According to the first law of thermodynamics, the critical strain of debonding was proposed for HTPB propellant.The overall effective modulus of composite solid propellant was determined based on the Mori-Tanaka method.The constitutive relationship of HTPB propellant is proposed for the uniaxial tension.The mechanical behavior of HTPB propellant reinforced by particles characterized with a log normal size distribution was discussed.Results show that the behavior of composite solid propellant is consisted of the initial linear elastic relation and nonlinear constitutive relation with interfacial debonding.With the increase of the vacuole fraction, the dilatation increases, but the effective properties of HTPB propellant decreases.For composite solid propellant, the proposed constitutive relation is easily used in engineering as long as the value of adhesion energy is available.