Glass fiber composite material is composed of glass fiber and resin, with high mechanical properties, low density, low cost characteristics. However, in the use and maintenance of aircraft, the moisture absorption of composite materials is inevitable, and can not be simulated well. This article analyzes moisture diffusion process through Tencate 7781 Glass fiber composite specimens conditioned in hygrothermal environment when effective moisture equilibrium is achieved. It indicates that mechanical property and glass transition temperature of wet composite would decline to different extent by contrasting with dry material, and the reason for descending is also analyzed preliminarily.