To study the tensile mechanical properties of the Nitrate Ester Plasticized Polyether (NEPE) propellant under varying confining pressure conditions, uniaxial tensile tests were conducted under different strain rates (from 0.0006667 s(-1)to 0.06667 s(-1)) and confining pressure conditions (from relative atmospheric pressure up 5.4 +/- 0.03 MPa) using a new-designed confining pressure testing machine. Scanning electron microscopy (SEM) was employed to observe the tensile fracture surfaces. The mechanical properties and damage processes of the NEPE propellant under varying confining pressure conditions were studied and analysed. The results indicate that the mechanical properties of propellant materials are remarkably influenced by the varying confining pressure conditions. The ultimate strain and maximum tensile stress increase with increasing confining pressure. Moreover, the maximum tensile stress present a linear-log relationship with the strain rate under various confining pressure conditions. Yet, the confining pressure has no obvious effect on the initial elastic modulus. The reason for the change of the mechanical properties of the NEPE propellant is that confining pressure delays the initiation and development of damage under the tensile loading. Based on the time-pressure superposition principle (TPSP), the master curve of maximum tensile strength of the NEPE propellant was constructed. Furthermore, the results can provide a theoretical foundation for the analysis of the structural integrity of propellant grains of a SRM under ignition conditions. Finally, a constitutive model considering compressibility of propellant materials can be constructed.