The works in recent decades on hygrothermal properties of polymer composite materials were summarized, including experimental research and theoretical methods. The hot/wet environmental conditions reduced composite properties mainly in two ways: properties of matrix vary with temperature and moisture concentration, and the inner stress distributions may be altered by hygrothermal state. The mechanical properties variation of composites with environmental conditions can be obtained through experiments. And it′s found that hygrothermal effects on matrix are particularly severe, indicating that the type of resin should be selected according to operating environment during the design procedure. Based on the above two ways of hygrothermal effects, numerical models were built to analyze the effects of humidity and temperature on composites. Many assumptions and simplifications were made in the models, which result in many limitations. Therefore, a multi-physical and multi-scale numerical model should be established to study the hygro/thermo/mechanical coupled effect.