Three-dimentional viscoelastic large deformation incremental constitutive equation was derived based on Total Lagrangian method. From materials property testing of composite modified double base(CMDB) propellant by dynamic mechanical analyzer (DMA),deformation,equivalent von mises stress and strain of free loading propellant in storage were obtained. The results show that the subsidence magnitude of solid propellant in the axis direction is about 0.16 mm. Outer diameter increases 0.04 mm and inner diameter is nearly unchanged. Stress between solid propellant and binder is about 11.8 kPa,which will not lead to dewetting. Balanceable time of the free loading propellant in long-term storage is about half a year. Thus the deformation of the free loading propellant grains in storage can be deduced by that of propellant stored for more than half a year.