Abstract: In the process of engine test, the outfield has repeatedly reflected the swing fault of the nozzle of the slow train. According to the fault situation and product structure, a fault tree is developed to investigate the events at the bottom of the fault tree one by one. Finally, cause of the slow train swing fault is located as follows: the plunger stuck, and larger friction with the shell leads to the unqualified work (stroke) of the self-locking plunger under slow vehicle (and military) condition, then leads to nozzle oscillation failure of the engine in the slow (military) state, and self-locking work is not normal. According to the fault causes, the corrective measures are formulated, and through sufficient mechanism analysis and effective test verification, the failure rate of the nozzle swing problem of the product is reduced.