Abstract: The cyclic moisture absorption and desorption behavior of CCF300 carbon fiber/5405 bismaleimid composites immersed in 71 ℃ distilled water were investigated. After three cycles of moisture absorption-desorption process, moisture diffusion coefficient and moisture uptake of CCF300/5405 composites in different cycles were studied. Using SEM, the change mechanism of absorption behavior of composites in the hygrothermal condition was discussed. The results show that the moisture absorption behavior of CCF300/5405 composites basically conforms the Fick's second law. After 14 days of moisture absorption, the composites reach to the saturated moisture level, and saturated moisture uptake is about 0.66%. Additionally, after cycles of moisture absorption process, the interface between fiber and matrix is damaged by moisture, which leads to a large number of voids and cracks. As a result, water diffusion coefficient increases significantly and saturated moisture uptake rises slightly. Furthermore, the damage is irreversible even after the moisture desorpion.