The moisture absorption behavior of quartz fiber reinforced epoxy panel/PMI core sandwich composites under different temperature and humidity conditions was studied. The quartz fiber reinforced epoxy composite panel, PMI foam and corresponding sandwich structure specimens were treated with different conditions for moisture absorption. Properties of these specimens were studied after moisture absorption. Results show that: firstly, the saturated moisture absorptivity and the moisture absorption rates of specimens are enhanced when they are soaked in water; secondly, when specimens are subjected to moisture absorption at 50 ℃, 60 ℃ and 70 ℃, saturated moisture absorptivity of the specimens dropped more at a higher temperature because of the mass loss in moisture absorption. It was also found that, when specimens are treated in a moisture condition below 60 ℃, the saturated moisture absorptivity of the PMI foam cored sandwich composites can be predicted by combining the moisture absorption rates of panel and the PMI foam counterparts.