The mechanical properties of composites are greatly affected by the environment. In this research, the glass fiber unsaturated polyester composite materials （GFRP） under different bending stress and submerged in hot water at a temperature of 80℃ is investigated. Loading 100 H (hours), 200 H, 400 H, 600 H, 800 H for testing under stresses equal to 0%（zero pressure）, 30%, 50% and 70% of the material ultimate strength are applied on samples. Then water absorption ratio is analyzed, and by experiment data of bending stress and modulus of elasticity in bending after three-point bending test, failure time, life curve, normalized strength degradation model and the fracture energy of composites are analyzed. Finally the change of material interface affected by the environment is observed with optical microscope and SEM, and the reasons of the change of material mechanics properties are explained.