Abstract: Main complex circumstances occurred during drilling process in Dabei-Keshen deep block and horizons have been analyzed in this paper.From the physical and chemical factors,the mechanism of sidewall instability has been studied.The results indicate the high proportion of clay minerals content and illite-smectite mixed-layer minerals,and developed micro-fractures are the main reasons of wellbore instability.Therefore,the collapse pressure has been calculat-ed.Meantime,according to the theory of rock mechanics,the fracture pressure has been determined,and then the safe mud density window of the block has been further determined.The research results has received good application effect in DB-10 well and greatly reduced the hole-diameter enlargement and stuck problems.