Abstract: Borehole stability has always been a problem that all oil fields have been trying to solve, and the cost caused by borehole instability is hundreds of millions yuan each year.With the overall development of Bozhong Block in Bohai Oilfield, deep wells and ultradeep wells are currently the main force for oilfield development.In Bozhong Block, The problems of borehole instability are serious, including leakage and borehole collapse, and they bring about great challenges to drilling operation.The collected cuttings and cores were experimentally tested and analyzed in consideration of the specific situations of drilled wells.Then, the distribution of rock collapse pressure was simulated by using MATLAB software.Finally, according to the theories related to rock mechanics, the borehole stability cycle was simulated by analyzing the water absorbing capacity of the shale over time and the change rule of rock strength in the immersion of drilling fluid.The research results provide the data and theoretical support for the drilling design optimization in the process of well drilling in Bozhong Block.