The complex waveform common mode current in variable frequency speed regulation system could affect the output accuracy and linearity of classical electromagnetic current sensor, which could result in the measurement error. Aiming at solving this problem, firstly analysis the formation mechanism, flow path and amplitudefrequency characteristics of common mode current, and then a magnetic modulation current sensor with simple structure and wide measurement range was proposed. The detection principle was described in details, and prototype design and experimental verification were conducted. The results indicated that the proposed sensor could measure complex waveform current, which could fulfill the requirements of common mode current detection.