In order to analyze the damage caused by the delayed firing fault to the external energy source automatic machine, a three-dimensional model of the automatic machine at the time of locking and firing was established. Based on the contact finite element theory, the nonlinear structural dynamic model of the automatic breech mechanism was established according to different breech opening times. The explicit time integration method was used to simulate the firing process, and the stress and failure of the main components of the automatic breech mechanism under the late firing state were analyzed, and the stress changes of the dangerous section and the damage of the cartridge case under the chamber pressure were obtained. The results show that the locking tooth root and the contact surface are damaged, the bottom of the cartridge case is broken, and the damage degree of the automatic mechanism is different under different delay time. The damage analysis of the late firing fault is consistent with the experimental results, which can provide a basis for the design of the late firing safety mechanism.