The invention discloses a relay protection method of LC parallel circuit detuning faults, comprising the following steps: a relay protection device samples current of parallel LC (a reactor and a capacitor) and samples total current flowing through the whole LC; reactor current is converted to equivalent capacitor current; by full-cycle Fourier numerical algorithm, amplitudes of equivalent capacitor current and realistic capacitor current are calculated, and amplitude of total current flowing through the LC is calculated; current amplitude ratio of an equivalent capacitor to a realistic capacitor is further calculated; and when amplitude of total current flowing through the LC is high enough, an alerting signal or trip signal is sent out by delay and idle contact of a tripping relay is outputted if current ratio exceeds upper limit and lower limit range set beforehand. In addition, the invention also provides a corresponding device. According to the invention, original characteristics of simple wiring and reliable method are retained. Meanwhile, stability of current ratio calculation results is improved, and sensitivity and rapidity of protection are raised.