In order to study the effects of the initial defects of composite solid propellant on the macroscopical mechanical properties, the engineering stress-strain curves of the propellants with different interface defect content were obtained through the uniaxial constant speed tensile test. It was found that the initial modulus and the tensile strength of propellants tended to decrease exponentially with the increase of the defect content. Then a mesoscopic model of propellant with interface defects was built and the mechanical response of defect interface was defined, and the mechanical response of propellants under the condition of uniaxial tensile load was simulated. Finally, comparing the simulation results with the experimental results, it is showed that the variation trend of the two is the same, and the error range is within 7.5%, which means that the method in this paper has some guidance values for the research of composite solid propellants with defects.