The causes leading to non-scheduled shutdown of turbo-generator sets in one power generalion Co Ltd from 2004 to 2008 due to relay protection failures,including causes from the secondary circuits,from relay protection,from excitation systems,and from man-made factors,etc.,have been analysed,and some recommendations being put forward,such as importance shoud be attached to management of the secondary circuits and the excitation systems,as well as periodic replacement of fuses on the primary side of volatage transformers,and importance has to be attached to periodic inspection and simulation test of quick-acting cut-off and spare automatic put-in devices etc.