Abstract: In view of the phenomenon that several alarms of loose parts occurred in the thermal test phase of a nuclear power plant during the overhaul, a probe was temporarily installed at a key location for field test, and the event location of the collected test data was conducted by analyzing the time difference of impact signal array waves reaching different sensors. The results show that the origin of the alarm event of the loose part is related to the change of thermal test-related state of the nuclear power unit. The event may be due to the spontaneous or indirect conduction of the swing limiter at that location. The successful diagnosis of this alarm event not only solves the actual engineering problem, but also is with great practical value to improve the detection interval of the existing monitoring system for loose parts.