In Yuanba area of northwestern Sichuan Basin, due to complex geological conditions, many complicated downhole accidents occur, such as the well leakage, the borehole collapse and the overflow, which would affect the drilling schedules. Using the GMI well stability software, based on the drilling data, the well logging and the geological information in Well Yuanba11, the mechanical parameters were calculated. Combined with the experiment data, the Young's modulus and Poisson's ratio were converted from dynamic values to static ones. In addition, the minimum and the maximum horizontal stresses were constrained and computed by the fracture information and the wellbore breakout method. Based on above results, the three pressure section, the well stability analysis and the mud scope were investigated, to provide a basis for the drilling design. The results indicate that in Jianmenguan Formation of terrestrial layer, the strata is characterized by fractures and well water permeability, where the fluid would easily be lost. Then in the air drilling section, due to the high and variable collapse pressure, the air drilling is not suitable. After the routine drilling replaces the air drilling, the mud density is lower and near to the pore pressure and the collapse pressure, which would lead to unstable borehole. So, many complicated downhole accidents would occur. While drilling in the depth of 4000m, the mud density is increased so that the complicated situations are mitigated. But in Xujiahe Formation, owing to the low mud density, the pore pressure is so high that the overflow would occur. In the marine section, the mud weight is higher, and actually, the formation pressure is lower. Therefore, the plastic flow of the salt layer would occur and sometimes the stickings may occur, which are distributed in Jialijiang Formation and Changxin Formation.