A MEMS (micro-electro-mechanical system) magnetic torsion micro-mirror fiber current sensor with composite structure is proposed. The sensor part is composed of a magnetic torsion mirror and a dual-fiber collimator. The structure of MEMS magnetic torsion micro-mirror and the principle of current sensor are introduced. Analysis based on the current sensor model shows that by choosing suitable distance between the collimator and the micro-mirror, signal distortion can be avoided. When the peak-to-peak value of normalized intensity is taken as the output signal of alternating current measurement, the measurement sensitivity changes with the measured value of electric current. When the measured current approaches 0, the sensitivity also approaches 0. Thus, one linear correction processing method for normalized intensity signal is proposed. With this method, a good linear response curve is obtained, and the current measurement sensitivity is not distorted by the current change. Finally, the correctness of theoretical analysis and simulation results are testified by experiments, from which the linear response sensitivity of normalized intensity reaches around 0.034/A.