Abstract: To understand the moisture absorption properties and its influence on composite, the adsorption energy of water molecules on the surface of nano MgO and SiO₂ particles was calculated by Materials Studio simulation software. The relevant mechanisms of moisture absorption were discussed and the changes of the dielectric properties for dry and wet composite were studied. The simulation results show that water molecules are mainly absorbed by the oxygen atom on the surface of oxide. Because water molecules can penetrate into the amorphous SiO₂ nanoparticles to form more adsorption so the nano SiO₂ has a greater amount of moisture absorption. Due to nano MgO has larger water molecules absorption energy compared with nano SiO₂, so the adsorbed water molecules in nano SiO₂ are more difficult to be removed. The nano MgO/low-density polyethylene(LDPE) and nano SiO₂/LDPE composites could absorb more water molecules than LDPE. The moisture absorption has a negative influence for the dielectric properties of composite. When MgO/LDPE and SiO₂/LDPE composite get wet, the conductivity increases. The presence of water molecules may destroy the tight structure and charge characteristics of the original interface region, and weaken the ability of the composite to suppress the carrier migration. When the test temperature increases to above 60℃, the adsorbed water molecules in MgO/LDPE and SiO₂/LDPE composite medium are basically removed, and the conductivity of composite recovers.