The thermal aging properties including the weight loss percent, the hardness, the gel fraction, the relative cross-linking density, the content of the plasticizers (BTTN and NG) were measured to analyze the aging characteristics of nitrate ester polyether (NEPE) propellant at three high temperature levels of 60 ℃, 70 ℃ and 80 ℃ under the thermal air condition. The results show that the hardness and the weight loss percent of NEPE propellant increases along with the aging time because of the volatilization and decomposition of the plasticizers, and the gel fraction and the relative cross-linking density decrease because of degradation of the binder matrix, and the contents of the plasticizers (BTTN and NG) decrease and are in conformity with first-order reaction. The reasons of NEPE propellant performances deterioration are the volatilization and decomposition of the nitrate ester plasticizers, the continued-curing reaction and the degradation of the binder matrix.