Borehole wall instability is one of the common difficulties in drilling process, especially in the mud shale interval drilling. In view of the complicated conditions of borehole collapse, block falling, drill pipe sticking and borehole diameter enlargement in Shanxi-Taiyuan formation mud shale drilling process in Henan Zhongmu shale gas block, the experimental analysis on the influence factors to borehole stability is carried out in the shale classification, pore permeability characteristics, microstructure, rock mechanics and in-situ stress by using the cores of well MY1. The analysis shows that the mud shale of Zhongmu block belongs to hard-brittle mud shale, the main factors that cause wellbore instability are strong heterogeneity, high rock brittleness and micro fracture development, which are easily lead to wellbore wall mechanical flaking, collapse and block falling. Through the reasonable control of drilling fluid density, optimization of the wellbore structure and drilling parameters and the development of low free water and strong plugging drilling fluid to improve the wellbore stability, good application effect is obtained.