The thermal aging properties, including the mechanical property, the gel property and interfacial property, were measured to analyze the aging characteristics of nitrate ester plasticized polyether (NEPE) propellant during aging process under 20% constant strain. The results show that the maximum tensile strength is decreased while the elongation at maximal has no obvious change. The performances deterioration of NEPE propellant is the invalidation of the strength. The gel fraction and the work of adhesion are decreased along with the aging time. The main aging mechanism is the degradation of the binder matrix and interfacial dewetting. There is a correlation between the maximum tensile strength and the gel fraction, the adhesion work. The equation is calculated to determine the relationship. The method is established to evaluate the macroscopical mechanical property by microcosmic property. Storage life of propellant under 20% constant strain is 8.3 years, while the value of deterioration of maximum tensile strength changes to 30%, it will reach the invalidation point.