Abstract: Geostress is one of the immediate causes of borehole instability; optimal azimuth of inclined hole and horizontal hole can not only avoids instability but also improves productivity. Based on the geostress and drilling fluid density of Bei28 Area, elasticity mechanism and improved Mohr-Coulomb strength criteria are applied to build borehole mechanical stability model, the model is developed to analyze the stable region and security drilling fluid density of Bei28 Area. Considering the influence of rock fracture on productivity, optimal azimuth of inclined hole and horizontal hole is determined. Geostress distribution is used to guide drill track, offering a new method to optimize well track.