The stability of diversion tunnel plug is very important for power generation and safe operation of hydropower station. Based on the summary and comparison of design and calculation methods for diversion tunnel plug at home and abroad, we carry out calculating analysis on the deformation, stress and stability of diversion tunnel plug under designed water level by 3-D numerical simulation analysis. Then, the progressive failure of plug from the parts to the whole is analyzed by over-loading safety factor method. The results show that the maximum displacement of plug is 1.1～1.4 mm and the shear strain on contact face is smaller with little extending range and failure under designed water level. According to the findings of over-loading safety factor method, the safety load factor that the plug can bear is 3.5～5.5 times of design water head, which can meet the standard requirements in safety and stability. And it is more reasonable and effective to adopt over-loading safety factor method to analyze the ultimate bearing capacity of diversion tunnel plug.