The invention relates to a loose part diagnosis method for a nuclear power station, which comprises the steps as follows: (1) event database building; (2) event signal analysis; and (3) positioning and quantitative calculation. The loose part diagnosis method for the nuclear power station is used for fault monitoring and diagnosis for the nuclear power station, timely analyzes the loose part alarm events after a loose part monitoring system gives off the alarm fault signal, confirms the features of the loose part and estimates the positioning quality of the loose part. The positioning error of the loose part events is 0.5m, the quality estimation error of the loose part is plus-minus 50% and the energy estimation error is plus-minus 50%.