The effect of humid and hot environment on the thermal safety and mechanical properties of RDX/AP-NEPE propellant was studied by means of TG, 5s delay explosion temperature test and unilateral tensile test. Results show that after 6 days of humid and hot aging under relative humidity of 70% and temperature of 75℃, the apparent activation energies of RDX/AP-NEPE propellant for three mass loss steps on TG curve are slightly lower, and the effect of water on the activation energy of AP decomposition reaction is more obvious, making the activation energy reduce by 9.3%. Its 5s delay explosion temperatures Tb before and after humid and hot aging are 303.7℃ and 302.7℃, respectively. With prolonging the aging time, the elongation rate and the tensile strength gradually decreases. Until sixth day, the elongation rate reduces from 106.0% to 36.7%, and tensile strength reduces from 0.631MPa to 0.541MPa. However, the apparent activation energy, 5s delay explosion temperatures,tensile strength and elongation rate of RDX/AP-NEPE propellant for three mass loss steps after 6 days of humid and hot aging under 75℃ do not change significantly. In early aging process, water has great effect on the mechanical properties of RDX/AP-NEPE propellant and activation energy of AP decomposition reaction, but it basically has no effect on its thermal sensitivity.