This invention provides an on-line monitor and fault diagnosis method for PWR nuclear power plant including the following steps: 1, data collection, 2, digit filtration and pre-process, 3, acquiring extremum points to carry out polygon process to the waveforms, 4, merging the extremum points and eliminating single peaks, 5, identifying character points of the waveforms in terms of upgrading, transmission and keeping to check current and waving amplitude, 6, judging step sequence and operation time and diagnoses fault, 7, comparing the consistency of four rod-beam operations of a sub-set to estimate them according to deviations and finish single-step analysis of the sub-set waveform, 8, collecting and displaying waveforms, storing effective data and storing action recordings.