The hydrothermal behavior of low curing temperature HS40 carbon fiber/cyanate ester resin composites under different hydrothermal environments as well as under three cycles of moisture absorption and desorption processes and continuous moisture for 60 d were investigated. The results show that the saturated moisture absorption of composites exposed to the environment of 70℃ water bath and 70℃/95% RH for 31d are 0.71% and 0.11%, respectively, which shows a good humidity resistance.The moisture absorption of composites at 70℃ water bath for the first 10d is 0.69%, and the moisture absorption of the second and the third immersion are 0.72% and 0.74%, respectively. Additionally, after three cycles of moisture absorption and desorption,the interlaminar shear strength of all the specimens reduces and the extent of decrease is more obvious as the increase of cyclic times. For the specimens after three cycles of moisture absorption and desorption, the interlaminar shear strength decreases by 50.77% compared with the dry specimens. For the specimens with continuous water absorption for 60d, the interlaminar shear strength decreases by 58.98% compared with the dry specimens.