In hygrothermal environment, water is easy to invade the epoxy resin aging, resulting in a decline in electrical insulation performance. In order to clarify the microcosmic mechanism of epoxy resin wet thermal aging, the effects of temperature and water content on the crosslinking system of epoxy resin were studied by molecular dynamics simulation technology. The results show that after water invasion, part of the free volume is occupied first. With the increase of moisture absorption, swelling and plasticization of the material occur, which causes the glass transition temperature to first increase and then decrease. On the one hand, high temperature intensifies the molecular movement, increases the energy of the system, and makes the system unstable. On the other hand, high water content enhances the interaction between epoxy resin and water molecules, and the combined action of the two aggravates the damp-heat aging of epoxy resin.