The quantitative characterization of the tensile strength of shape memory polymers (SMPs) at different temperatures has always been an important research topic. In this study, the critical failure energy density of SMPs including the strain energy density, potential energy and kinetic energy of atomic motion per unit volume is first introduced. Then, based on the equivalent contribution of these energies on material failure, a temperature dependent tensile strength (TDTS) model considering the corresponding physical mechanism for SMPs is established. The model provides the quantitative relationship among temperature, Young's modulus, hardening index and the tensile strength of SMPs.Meanwhile, the predicted results of the proposed model are compared with the available TDTS of SMPs, and the agreement between theory and experiment is satisfactory. In addition, the influencing factors of tensile strength and their variation with temperature are analyzed. This work contributes the novel insight for the theoretical predictions on the TDTS of SMPs, which is helpful for the high temperature strength evaluation and property optimization.