Abstract: The hygrothermal properties of high-temperature curing epoxy 5228A resin and carbon fiber/ epoxy 5228A resin composite laminate were investigated under three different hygrothermal conditions,  i.e. boiling water,  water immersion and relative humidity 85% at 70℃. The effects of hygrothermal condition on the interlaminar shear strength of the carbon fiber/epoxy resin composite laminates were studied. The hygrothermal aging mechanism was analyzed based on the moisture absorption,  physical and chemical properties,  the mechanical properties property of epoxy resin and moisture stress. The results show that the degradation degree of the interlaminar shear strength mainly depends on the moisture content,  and the degradation degrees under different hygrothermal conditions are almost identical at the same water absorption content. The chemical change and interface micro-cracks have not been found in the composite laminate under the three hygrothermal conditions. The resin plasticization with water absorption and moisture stress due to different moisture expansion between fiber and resin is the main reasons for the degradation of the composites laminate.