For resolving problem of closing breech ahead of time of certain type naval gun, this paper analyzes the causes of fault based on principle of fault tree, establishes fault tree for closing breech ahead of time, obtains the minimum cut set of fault causes by using the downward method, defines each base event of this failure phenomenon, respectively analyses each base event through local overhaul inspection and theory simulation to find out the weak unit of the mechanism for opening and closing breech. Combining with the product structure, physical quality and test fault phenomenon, this paper makes conclusion that the rupture of closing breech spring is the main cause to result in this failure, and this paper carries out the fault recurrence and improvement validation with respect to the base event. The analysis result shows that it is practical to diagnose the fault of naval gun with the fault tree analysis method, and that this analysis method is helpful in application and expanding of fault tree analysis method in fault analysis of naval gun.