The critical technology of condition-based maintenance of relay protection is analyzed，and the on-line state detection method of main relay protection secondary circuits such as AC secondary circuit，switching value input secondary circuit and breaker operating secondary circuit are studied.For secondary AC circuit on-line status detection，a solution of making comparative analysis between its AC input value and reference value is proposed.Specific on-line state detection schemes each for the single and dual configured relay protection devices are proposed，respectively.As for on-line status detection schemes of switching value input secondary circuit，an abnormal status detecting scheme using microcomputer is proposed for certain types of switching value input secondary circuit.With respect to on-line status detection of the breaker operating secondary circuit，a detailed state detecting circuit is designed，and the proposed switch on/off export contact reliability detecting method based on double relay contacts in series is analyzed at length.The proposed method is simple and practical，and can be better to realize the real-time on-line detection of switch on/off export contact action.