With the purpose of investigating the effects of confining pressure and aging on the mechanical prop-erties of Hydroxyl-terminated polybutadiene(HTPB)based composite solid propellant,tensile tests of thermal accelerated aged propellant samples under room temperature and different confining pressure conditions were performed through the use of a self-made confining pressure device and conventional testing machine.Afterwards,the maximum tensile stress σm and the corresponding strain εm for the propellant under different test conditions were obtained and analyzed.The results indicate that confining pressure and aging can significantly affect the mechanical properties of HTPB propellant,and the coupled effects are very complex.On the one hand,the stress σm increases as a whole when confining pressure becomes higher or thermal aging time rises.Besides,this stress is more sensitive to aging with increasing confining pressure.There are almost three regions in the stress increments(σmP-σm0)/σm0 and thermal aging time curves for HTPB propellant.The maximum value of the stress increment(σmP-σm0)/σm0 for the propellant is about 98％at 7.0 MPa and 170 d.On the other hand,the strain εm decreases with increasing thermal aging time under the whole confining pressure conditions.However,the vari-ation of this strain with confining pressure is more complex at various thermal aging time,which is different from that of unaged solid propellant in previous researches.In addition,this strain is slightly less sensitive to aging as the confining pressure increases.Furthermore,there is also a critical confining pressure in this investigation,whose value is between 0.15 MPa and 4.0 MPa.Beyond this critical pressure,the trends of the stress σm and the corresponding strain εm all change.Moreover,there are some critical thermal aging time for the stress increment(σmP-σm0)/σm0 and strain increment(εmp-εm0)/εm0 of HTPB propellant in this investigation,which are about at 35,50 and 170 d.Finally,based on the twin-shear strength theory,a new modified nonlinear strength criterion of thermal aged HTPB propellant under confining pressure was proposed.And the whole errors of fitted results are lower than 6％.Therefore,the proposed strength criterion can be selected as a failure criterion for the analysis the failure properties of aged HTPB propellant under different confining pressures,the structural integrity of solid propellant grain and the safety of solid rocket motor during ignition operation after long periods of storage.