In order to acquire dynamics response of certain type automatic mechanism during gun fire, modeling and simulation for virtual prototype of gun automatic mechanism were investigated and studied. Based on the analysis of automat configuration and mechanism movement, virtual prototype model was set up by means of ADAMS software package. Firing dynamic simulation of the automatic mechanism is carried out under the conditions of repeating shots by comparing the test results with the simulated data. A series of kinematics curves of the automatic mechanism are obtained and analyzed, and the results show that the virtual prototype is feasible, which built the groundwork for rectifying the dynamic characteristics and improving the campaign efficiency of this rapid-fire mortar.