The invention relates to an MEMS magnetic field sensor of a folded beam structure and a preparation method. The sensor includes a metal T-shaped folded beam processed on a substrate and fixed in the center. An electrode formed by the folded beam and an electrode on the substrate form a variable capacitor. The principle of the MEMS magnetic field sensor of the folded beam structure is that a metal coil processed on the T-shaped folded beam structure is subjected to lorentz force after current is introduced, thus the beam bends and deforms, thereby changing an interval of the two electrodes, and realizing change of a capacitance value. The magnitude of a magnetic field can be measured through detection of capacitance variation. The folded beam structure can realize effective support for a movable polar plate, and provide a smaller equivalent elastic coefficient compared with a traditional structure, and compared with a traditional torsion beam structure, various residual stress in a sensor processing process can be reduced, sensitivity of the sensor can be improved, and power consumption can be reduced. Since the T-shaped folded beam structure of the sensor can achieve a function of releasing stress, the service life of the sensor is prolonged.