The invention discloses an alternating current servo drive major loop based on three electrical levels of an MOSEFT. According to the major loop, on the basis of the increased level number of a three-electrical-level MOSFET main circuit topological structure, the voltage waveform input by a motor is improved, the deformation of the motor voltage waveform is reduced, the harmonic wave of motor current is decreased, and temperature rise in the motor is reduced; meanwhile, the stress of a diode Du/ a diode Dt, which is performed on switch devices, is greatly decreased, an insulated part of a motor rotor winding can be effectively prevented form being penetrated, and the EMI characters of an overall servo drive can be improved; the switching frequency reaches 50 KHZ, and the integral propertyof servo control is significantly improved, particularly the dynamic property and the velocity response bandwidth.