Abstract: The complex stress and solid?liquid interaction in drilling cause hydration diffusion and swelling in surrounding rocks, which changes the stress field and mechanics parameters of the rocks and leads to the borehole instability of shale. Based on experiments, the hydration changing law of surrounding shale with time and space as well as the damage weakening rule of elastic modulus and compressive strength of the rocks with water content are discussed in this paper. Then with the consideration of strength?deformation and damage theory, the damage?evolution equation and limit equation after the invasion of water into shale are deduced, and accordingly the collapse cycle of borehole wall is determined. Applying this method to Sulige gas field shows that its shale borehole wall collapse cycle is about 6.8 days