Hygroscopic behavior was investigated with different scarf patch repair methods of GFRP laminates, then hygroscopic law and damage behaviors were gained. Main factors leading to decrease of hygroscopic property and interlaminar fracture were investigated and analysed through the in-plane shear properties tests and morphology observation, whch show that hygroscopic behavior increases with the increase of porosity, while the interlaminar fracture tendency decreases, the important factors influencing hygroscopy and interlaminar cracks is the porosity of GFRP laminates. Step ratio has a great influence on intensity of laminates, selecting the best step ratio is a good way to improve strength retention and decrease the strength reduction influenced by hygroscopic behavior. The investigation also confirms that hygroscopic behavior can seriously weaken fibre resin interface bonding state and the hygroscopic behavior is one of the main reasons which lead to the significant reduction of in-plane shear strength of laminates after moisture absorption.