The invention discloses a failure diagnosis method for neutron detectors of reactors. The failure of measured values of the detectors is diagnosed at three stages with a direct comparison method, an inter-measured-value comparison method and a measured value and responding reconstructed value comparison method for the measured values of the detectors; due to the combination of the three judging method, the failure diagnosis method for the reactors can effectively diagnose the complete failure of the detectors, can make accurate diagnosis for the deviation failure of the measured values of the detectors from normal values and has the capacity of distinguishing the detector failure from the local power oscillation; by removing the measured values of the failed detectors, the measured values of the failed detectors are not used by an online monitoring system for reactor core power distribution, and reliable input parameters are provided for the online monitoring system. The failure diagnosis method for the detectors is not limited by the type of the detectors, an online monitoring method and the failure type of the detectors, and has the advantages that the failure diagnosis time is short, the diagnosis is quick, and diagnosed results are accurate and reliable.