The lack of long-term stability has all along been baffling the popularization of the optical current transformer（OCT）. Based on the conventional OCT， comparative measurement OCT introduces an invariant magnetic field generated by a permanent magnet as the reference source. The final current value is obtained by comparing the current magnetic field with the reference one， realizing compensation for both the invariance of Verdet constant and linear birefringence at the same time and making the application of the OCT in industry a reality. In view of the new sensor structure of the comparative measurement based OCT， a brand new signal demodulating method of dual-source and dual-output is designed. The linear test results show that the signal demodulation is valid and that the accuracy of the OCT proposed is up to the 0.2 class accuracy requirement in IEC60044-8.