A relay protection method against LC parallel circuit (L1, C1; L2, C2; L3, C3) detuning faults, comprising the following steps: sampling separately the current through each of the reactor, L, and the capacitor, C, in said LC parallel circuit to provide the current (i L) of the reactor and the current (i C) of the capacitor, and sampling a total current (i LC) flowing through the whole LC parallel circuit; converting the current (i L) of the reactor into a current (i Ceq) of an equivalent capacitor, C eq; calculating amplitudes (I Ceq' I C) of the current of the equivalent capacitor and of the current of the capacitor of the LC parallel circuit and calculating an amplitude (I LC) of the current flowing through the whole LC parallel circuit; calculating a current amplitude ratio (Ratio(n)) of the amplitude of the current flowing through the equivalent capacitor to the amplitude of the current flowing through the capacitor of the whole LC parallel circuit; and sending an alarm signal or a trip after a setting time delay when: a) the amplitude of the total current flowing through the LC parallel circuit exceeds a reference value, and b) the calculated current amplitude ratio exceeds a preset upper and lower limit range, wherein step specifically comprises: converting the current of the reactor into a current of an equivalent capacitor by differentiation, wherein the formula adopted is: i Ceq n = n CT, L n CT, C LC i L n − 2 i L n − 1 + i L n − 2 T s 2 + RC i L n − i L n − 1 T s wherein n CT,L and n CT,C are a current conversion ratio of current transformers of the reactor subcircuit and the capacitor subcircuit in the LC parallel circuit, respectively, R is an equivalent resistance value of the reactor subcircuit and T s is a sampling time interval, and n is a serial number of the sampled current signals. sampling separately the current through each of the reactor, L, and the capacitor, C, in said LC parallel circuit to provide the current (i L) of the reactor and the current (i C) of the capacitor, and sampling a total current (i LC) flowing through the whole LC parallel circuit; converting the current (i L) of the reactor into a current (i Ceq) of an equivalent capacitor, C eq; calculating amplitudes (I Ceq' I C) of the current of the equivalent capacitor and of the current of the capacitor of the LC parallel circuit and calculating an amplitude (I LC) of the current flowing through the whole LC parallel circuit; calculating a current amplitude ratio (Ratio(n)) of the amplitude of the current flowing through the equivalent capacitor to the amplitude of the current flowing through the capacitor of the whole LC parallel circuit; and sending an alarm signal or a trip after a setting time delay when: a) the amplitude of the total current flowing through the LC parallel circuit exceeds a reference value, and b) the calculated current amplitude ratio exceeds a preset upper and lower limit range, wherein step specifically comprises: converting the current of the reactor into a current of an equivalent capacitor by differentiation, wherein the formula adopted is: i Ceq n = n CT, L n CT, C LC i L n − 2 i L n − 1 + i L n − 2 T s 2 + RC i L n − i L n − 1 T s wherein n CT,L and n CT,C are a current conversion ratio of current transformers of the reactor subcircuit and the capacitor subcircuit in the LC parallel circuit, respectively, R is an equivalent resistance value of the reactor subcircuit and T s is a sampling time interval, and n is a serial number of the sampled current signals. a) the amplitude of the total current flowing through the LC parallel circuit exceeds a reference value, and b) the calculated current amplitude ratio exceeds a preset upper and lower limit range,