Abstract: Measurements of in-core detectors, as one of the most important inputs for a reactor core on-line power distribution monitoring system, seriously affect the on-line monitoring results. Therefore, the diagnosis and processing of detector failure are necessary. Harmonics expansion method is employed to the on-line monitoring calculation of reactor core power distribution. To diagnose failure of detectors, three methods direct method, comparison of measurements method and comparison of reconstructed responses methodare used in combination. Based on these methods, diagnosis and processing of detector failure have been added into NECP-ONION, an in-house online monitoring code developed by NECP Lab. Benchmark for Evaluation and Validation Reactor Simulations(BEAVRS) has been used for code validation, especially for the diagnosis and processing of detector failure. Numerical results show that, the combination of these three methods is not only effective to the complete failed condition, but also useful for measurements deviation of normal value. In addition, NECP-ONION is functional to distinguish detector failure or local power oscillation. This could reduce the misdiagnosis for detector failure. For the processing of detector failure, it has been observed that single detector failure or a small number of detectors failed has limit effect on the monitoring system NECP-ONION.