Moisture measurement, shear strength testing and morphology observation of glass fiber reinforced epoxy resin composites (GF/E-51) specimen immersed in water were carried out in this paper. Experimental results show that the maximum moisture content of the specimen in water of 50℃ was increased by 145% than that in water of 30℃, while the diffusivity coefficient increased by 47.6% and the average water absorbing rate increased by 300%. Increasing temperature accelerated the transformation of water absorbing process from the Fick law based to the No-Fick law based. Increasing pressure, however, decreased the process as the pressure restrains the swelling of the GF/E51 in water. Sixty days after the specimen was immersed in water, the shear strength of all specimen will reach the minimum values while the shear strength of the specimen in water of 5atm and 30℃ is 3.97% higher than that of the specimen at 1atm and 30℃， and 5.17% higher than that of the specimen at 1atm and 50℃, respectively.