Abstract: In this paper, the drilling problems and the borehole stability of Well BZ-A3 are analyzed and studied. There are a lot of problems during the drilling process of Well BZ-A3, which resulted in two sidetracking and corresponding economic losses. In this paper, through reviewing the existing logging data, establishing the rock mechanics model of the borehole stability and analyzing the borehole stability, it is found that due to the use of low-density drilling fluid, the damage range of Well BZ-A3 and two sidetracking is large, and the length of borehole collapse arc is greater than 90°, which leads to large shale blocks falling off from the borehole or even borehole collapse, which leads to serious sticking problems. The Eaton method is used to calculate the formation pore pressure. The analysis results show that there is obvious overpressure in Dongying Formation and Shahejie Formation. The main overpressure mechanism is undercompaction and hydrocarbon generation. The maximum pore pressure is 1.4～1.5 g/cm³. Based on the pore elastic model, the horizontal low stress is calculated, and the maximum horizontal principal stress direction is N65°E, which is consistent with the geological structure of the block and the world diagram of the crustal stress. This paper finds out the mechanism of borehole instability and sticking in Well BZ-A3, and puts forward suggestions for drilling mud density, so as to help avoid such accidents from happening repeatedly drilling operations of adjustment wells in this block in the future .