Abstract: To solve the wellbore instability problem of a deep sidetracking well in hard-brittle mudstone strata of west China,this paper indicated the mechanism of wellbore instability considering the effects of both minerals and drilling mud on rock compressive strength.Based on failure condition of wellbore,a model of well instability was built considering the contributions of bedding plane occurrence,in-situ stress,and well trajectory,etc.The calculations of this model for the sidetracking mudstone segment in Well TKX-CH showed that the collapse pressure is 1.11 kg/L.Serious rock-collapse happened in practice when drilling mud was 1.12 kg/L.This situation improved as the drilling mud density decreased to 1.1 kg/L,when the deviation angle reached to 58°.When the sidetracking orientation and deviation angle are constant,the density of drilling mud to keep well stable increases as the ratio of stress difference increases and as exposure time of mudstone inside drilling mud increases.Therefore,in order to decrease the risk of borehole instability,in-situ stress,bedding planes and exposure time of mudstone should be considered systematically in drilling design.