The invention discloses a phase current offset compensation method and a device for an AC synchronous motor. The phase current compensation method is used for compensating phase current offset of the AC synchronous motor caused by rotation transformer factors. The phase current compensation method comprises steps: three-phase current signals of the AC synchronous motor are acquired, the three-phase current signals are converted to two-phase current signals, either direct-axis current signals or quadrature-axis current signals included in the two-phase current signals are used as to-be-processed signals, harmonic analysis is carried out on the to-be-processed signals, and when certain-order harmonic for the to-be-processed signals obtained through harmonic analysis does not meet a harmonic content condition, the certain-order harmonic is compensated until the certain-order harmonic meets the harmonic content condition. The phase current offset compensation method and the device for the AC synchronous motor can effectively solve the problems of heating of a motor and a motor controller and ripple of motor torque caused by phase current offset when the motor operates at a high speed.