Abstract: Wellbore instability has always been a restraint on the high-efficiency production in the Bohai Oilfield. With the implementation of the deep-layer-oriented hydrocarbon exploitation strategy, drilling operations are performed towards increasingly deep and tight layers, making the role of wellbore stability more prominent in supporting the safe and cost-effective drilling. In order to reveal the microscopic mechanisms of wellbore instability and precisely predict the collapse pressure interval, a discrete element model was developed for wellbore stability analysis. With this model, the effects of multiple particle contact microscopic parameters on the formation mechanics were analyzed. The sidewall breakout width criterion was introduced to define a wellbore instability, and the effects of the wellbore fluid column pressure on the failure form of the sidewall rocks were identified. The results show that after the bonds between particles representing formations break, the stress re-distribution occurs due to the resultant overall rigidity reduction, and the load acting upon the original damaged zone is transferred to the un-damaged zone, which then triggers successive local failure that progressively propagates toward the deeper zone until a stable failure zone is formed. Therefore, the lower limit of an appropriate collapse pressure equivalent density should be able to maintain the stability of the wellbore with moderate failure, and in the case of a narrow safe mud density window, a drilling fluid density approaching the lower limit may be used in practice. Field applications in 12 wells of the BZ13-1 Oilfield delivered a reduction of the drilling complexity rate of 67%, which validates the accuracy of the presented discrete element model for wellbore stability analysis.