Flushing fluid’s density is very important in the drilling project. Large density is helpful for wall protection and dreg suspension but will increase drilling resistance at the same time; while too small density will reduce the capability of dreg suspension and influence the stability of wellbore. Based on the mechanical analysis on the borehole stability, simulation device was established to simulate the stability of borehole wall. Meanwhile, the stability of wellbore was also simulated by finite element method to obtain the reasonable safe density window of flushing fluid. Comparison results show certain feasibility of simulation using ANSYS finite element analysis software to determine the safe flushing fluid density window. Using finite element simulation analysis not only can reduce the cost of flushing fluid test in earlier stage, but also can provide the safe density window of flushing fluid, which has practical reference value to optimize the density of flushing fluid.