Abstract: In order to study the performance evolution of carbon fiber reinforced epoxy composites under moisture absorption coupling four-point bending load, a four-point bending loading fixture was self-designed to provide the load condition, and 5 times concentration of seawater simulating moisture environment was coupled to simulate the service condition. The damage initiation and propagation process of composite under mechanical loading were monitored in situ by self-designed crack observation system. The effects of different pretreatment conditions on mechanical properties and damage modes were compared and analyzed. The loading area and equal bending moment area of composite under four-point bending pretreatment conditions were evaluated by four-point bending, three-point bending and short beam shearing test, respectively. The results show that the four-point bending property is decreased after pretreatment, the composite is damaged at one end of the loading at first, and then the load continues to decline for several times until the final failure occurs. There is no obvious change in the short beam shearing performance of the equal bending moment section. The three-point bending mechanical test results show that the pretreatment has no significant stress concentration effect on the equal bending moment area. Thus, the pretreatment conditions had a significant effect on the loading area but insignificant on the equal bending moment area. Multiple fracture propagation was observed on the loading area during the failure process, accompanied by interlayer crack propagation.