Abstract: During the outage of a nuclear power plant, a loose parts impact test was carried out on the steam generator, and the main defect modes and treatment methods of the loose component monitoring system (LPMS) in long-term operation are proposed. The blind area in the function of fault self-checking in LPMS system is given, and the response characteristics of the defective channel to the impact signal of the loose parts are analyzed. Poor channel contact, electric charge accumulation and multi-channel signal interference are the main factors that cause the signal distortion. The accumulation of electric charge causes the static blocking of the signal channel; the signal interference between multiple channels is an important cause for false alarms and channel overload and disconnection.