In order to find an effective and accurate life calculation method, the certain type automatic rifle is taken as an example to analyze the strength life of the locking mechanism. The locking mechanism is analyzed by using ABAQUS finite element software and the data results are analyzed by rain-flow counting method. The local stress strain is calculated by using the cyclic stress-strain curve equation and the modified Neuber method. According to the fatigue damage law and the Miner linear fatigue cumulative damage theory, the fatigue life of the gun locking mechanism is calculated. The results show that the method can save the cost and test time of the firearm mechanism in the development process. Compared with the existing methods, it can get more accurate prediction results, and has a certain reference value for the study of the strength and life of the automatic rifle locking mechanism.