A high-accuracy optical current sensor (OCS) utilizing the Faraday effect is proposed. By making use of the characteristics of wide bandwidth of OCS and according to superposition theory, a reference sinusoidal wave ac current with stable amplitude and different frequency from measuring ac current is added to the same one SF-6 glass bulk ring which is used as sensing element. Then comparative measurement is carried out and the ratio of two output signals of the Faraday rotation is shown independent of ligh intensity, Verdet constant, and temperature. Experiments performed at room temperature to 60 degrees C show that good linearity and accuracy can be obtained by using this method. The application of this method in actual environment is discussed.