Humidity aging experiments of investigating effects of environmental humidity on NEPE propellant were carried out. Mechanical properties, humidity absorption ratio, and variation of stabilizers content of NEPE propellant were measured. Results show that absorption humidity seriously affects mechanical properties of the NEPE propellant. Tensile strength may decrease by 70% of initial value in 85% RH condition. The NEPE propellant′s equilibrium humidity is very low,and the propellant can absorb humidity slightly in 11% RH condition. Humidity aging rate is relative to exposed surface area and scale of specimens. The mechanical property loss resulting from short-term humidity aging can be recovered mostly by dehydration in dry air. Humidity accelerates chemical aging obviously in NEPE propellant at elevated temperature. It suggests that it is necessary to take humidity effects on chemical reactions into account in long-term storage evaluation.