Abstract: It is crucial for drilling the horizontal sections of horizontal wells in Changing Block to uncover the reasons for borehole instability in Longmaxi and Wufeng fromations and put forward the countermeasures of drilling fluid technology to strengthen the borehole stability. Based on X-ray diffraction, scanning electron microscopy, shale swelling, and rolling dispersion tests, the mechanism for the borehole instability of complex strata was revealed and a coordinative borehole stability method was proposed, i.e., "strengthening plugging of micropores, inhibition of filtrate invasion, and retardation of pressure transfer". With the test devices such as sand-bed filtration testers, high-temperature and high-pressure filtration simulators, and microporous membranes, the treatment agent of oil base drilling fluid in Changning Block, which was dominated by plugging agents, was selected. And a drilling fluid system with strong plugging property, which was suitable for Changning Block, was developed. The system had a temperature resistance of 135 ℃, salt resistance of 10%, calcium resistance of 1%, poor-soil contamination resistance of 8%, bearing capacity of 5 MPa for 400 μm fractures, and filtration loss of 0 for both 0.22 μm and 0.45 μm microporous membranes. Its plugging effect was significant, and its comprehensive performance was better than that of ordinary drilling fluid. The drilling fluid was applied to more than 10 wells in Changning Block and borehole instability was not encountered in the horizontal sections of Longmaxi and Wufeng formations. In comparison with the drilled wells applied with the conventional drilling fluid technology in the same block, the hole diameter enlargement rate of complex formations was reduced by 10.82% on average, and the construction cycle was shortened by 4.5 days on average. The research results demonstrated that the oil base drilling fluid technology with strong plugging property can effectively solve the borehole instability problem in the horizontal sections of Longmaxi and Wufeng formations in Changning Block, and it is worthy of promotion and application.