Considering the propellant of solid rocket motor with tube grain is in the state of superimposed pressure after ignition, the effect of pressures, temperatures, and tensile rates on the mechanical properties of solid propellant were investigated together. A test system with wide temperature-ambient pressure range was developed to investigate the effects of superimposed pressure on the performance of propellant. The uniaxial constant tensile rate tests for a certain HTPB propellants under different superimposed pressures and temperatures were performed at different tensile rates. Research shows that there are no apparent dewetting points in the propellant stress-strain curves under superimposed pressures. Moreover, the performance of propellant under superimposed pressure related to propellant itself such as composition of the propellant was evaluated by comparing to another propellant. Relevant methods and conclusions can provide a reference for structural analysis and storage life prediction of solid rocket motors.