Abstract: The wellbore safety factor is used as a criterion to determine the wellbore instability area, and with the Monte-Carlo simulation, the instability area is calculated. To consider the wellbore collapse risk, the influence of the mud density, the deviation and the azimuth on the wellbore instability area is analyzed, and the effect of the well trajectory parameters on the wellbore instability in different in-situ stress fields is evaluated by the method of calculating the instability area proposed in this paper. The evaluation results agree with the actual engineering practice, which shows that this method could be used to evaluate the characteristics and the severity of the borehole instability.