Abstract: In this article, the No.9 coal seam of the Carboniferous upper Taiyuan formation is taken as geological background, the stress and deformation of the surrounding rock under three kinds of drilling fluid column pressure in under-balanced drilling was analyzed by using the finite element software COMSOL Multiphysics. The study shows that the stress and deformation of surrounding rock in the horizontal wells in vertical direction of the study block is the most severe. There is a sudden change of stress in the surrounding rock, which is easy to produce tensile and sheared crackles and lead to fracture or instability of the surrounding rock. The stress distribution and deformation of the surrounding rock are related to the drilling fluid column pressure, the smaller difference between the liquid column pressure and the reservoir pressure is, the more stability of the wellbore will.