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With the development of HVDC project in State Strong Smart Grid, the Faraday magneto-optical effect optical current transformer has been paid more and more attention to. However, the signal to noise bands overlap in original Faraday effect DC optical current transformer causes its measurement accuracy to decrease. Therefore, this paper presents a Faraday magneto-optical effect DC optical sensing method based on modulation and demodulation. Moreover, a new Faraday effect DC optical current transformer measurement system is deduced and designed on this basis. Subsequently, this paper mainly studies and designs an AC modulated optical source. With data acquisition and analysis of several experiments by LabVIEW, the results prove that the optical source can steadily generate modulated light intensity. Based on the AC modulated optical source, combined with the DSP technology, a complete DC optical current measurement system is formed. The practicality of the designed DC optical current transformer system has been tested by experiments in this paper.