Abstract: The flexural properties of carbon fiber/epoxy (T700/TR1219B) composite were investigated under different hygrothermal conditions, including single moisture, temperature and coupled moisture-temperature conditions. Combined with fracture morphology and surface roughness, the degradation mechanism was analyzed. The experimental results show that the flexural properties decrease significantly under hygrothermal conditions. With the moisture absorption rate of 2%, the flexural strength decreases sharply from 1 440.60 MPa to 1 081.07 MPa due to the interfacial debonding caused by water absorption. The flexural properties decrease with temperature increasing and exhibit a steep fall in the range of glass transition temperature T_g. The flexural modulus and flexural strength decrease by 71.18% and 93.32% at 180℃, respectively, caused by resin degradation under high temperature. The coupled moisture-temperature condition results in the steep fall of properties at lower temperature range. The residual strength model is established and verified concerning the effects of hygrothermal conditions. The life-time estimation of T700/TR1219B composite is realized by introducing the aging time and environmental equivalence.