A study on the hygroscopic characteristics of domestic T800 - grade carbon fiber - reinforced thermoplastic composites and glass fabric - reinforced thermoplastic composites was carried out. The hygroscopic curves of the materials in an environment of (70 °C, 85% RH) and in water immersion at 70 °C were obtained. Moreover, the effects of different hygroscopic times on the bending properties and short - beam strength properties of the materials were investigated. The results showed that, according to the current hygroscopic equilibrium criterion, the hygroscopic amounts of the materials in different hygroscopic environments varied significantly, and the mechanical properties of the materials were also significantly affected by the hygroscopic methods. Based on this, suggestions for the hygroscopic methods of thermoplastic composites were put forward.