Abstract: Serious wellbore collapse problems occurred during drilling through the shale layers of Hanjiang formation and Zhujiang formation in eastern part of South China Sea, which had great influence on drilling speed and cause huge economic losses. Therefore, the wellbore instability mechanism of the shale formation in this region has been analyzed, and the water absorption and diffusion coefficient of shale through laboratory tests has been determinated. In addition, the influence of shale hydration on mechanical parameters, such as compression strength and elastic modulus has been analyzed. Finally, based on the Mohr-Coulomb failure criterion, the change of collapse pressure with time after the shale formation is opened has been determinated. The research results are very significant to solve the shale collapse problem in this region.