To investigate the impact of humid-thermal coupled environments on the structural stability of HNIW crystals and its underlying mechanism， accelerated aging tests， in-situ X-ray diffraction， and molecular dynamics simulations were employed to study the phase transition behavior of HNIW under various humid-thermal conditions and its intrinsic causes. It was found that the crystal transition temperature of HNIW in humid-thermal environments significantly decreased compared to that under thermal stimulation alone， dropping from 135 ℃ to 80 ℃. The particle size effect of HNIW led to multipath phase transition behaviors， namely ε→γ and ε→α， indicating that differences in crystal characteristics are key factors contributing to the multipath phase transitions in HNIW. Among these， the ε→γ transition rate was the fastest in ultrafine HNIW. Under humid-thermal conditions below 70% RH， the phase transition of HNIW was not significant， but microstructural damage resulted in reduced stability of the crystal phase structure and a decrease in the thermal crystal transition temperature. Combined with theoretical calculations， it is generally concluded that a hot and humid environment can induce liquid-solid interfacial reactions in HNIW. Water molecules， through a mechanism of surface micro-dissolution-induced nucleation and growth， promote the crystal form transformation of the explosive. This process leads to the embedding of water molecules into the internal structure of the HNIW crystal， resulting in the formation of the α crystal form. However， this embedding mechanism is selective； for ultra-fine particles， it directly induces transformation into the γ crystal form， which is stable at high temperatures.