A conventional triaxial confining pressure test was designed to suit the working environment of propellant grain. The mechanical properties of the double-base propellant under varied confining pressure conditions were studied and analyzed. The results show that confining pressures have a pronounced influence on the mechanical properties of propellant materials. The yield value and compressive strength of propellant material increase as the confining pressure increases. The propellant material shows great ductility before it is destroyed, the ductility of propellant material increases with an increase in the confining pressure. The yield values and failure values of the propellant materials show linear relationships with the confining pressures. According to the Mohr-Coulomb theory, the cohesion and internal friction angle of the double-base propellant material were obtained. The influence factors of cohesion and internal friction angle of the double-base propellant material are analyzed. The failure process and mechanism of propellant materials under varying confining pressures were studied. These projects have contributed to the understanding of the innate character of propellant materials and explain the damage and destruction of the structures and components. With this information, measures can be taken to improve the quality and structural performance of propellant grain. The information collected herein may lay the foundation for yield criteria and viscoelastic-plastic constitutive model of propellant materials.