The embodiment of the invention, which belongs to the field of permanent-magnet synchronous motor control, provides a low-frequency control method and device of a permanent-magnet synchronous motor, a compressor and an air conditioner. The method comprises: after starting of a permanent-magnet synchronous motor, an electromagnetic torque is obtained and a first mechanical position angle is determined; according to the electromagnetic torque and the first mechanical position angle, an instantaneous load torque of the permanent-magnet synchronous motor is determined; a torque compensation current is determined by using the determined instantaneous load torque and a mapping relationship between an instantaneous load torque and a torque compensation current; and torque compensation is carried out on the permanent-magnet synchronous motor based on the torque compensation current. According to the low-frequency control method and device, the electromagnetic torque and the mechanical position angle are used as inputs; and compared with the mechanical rotation speed, the high-frequency noises are reduced; and no filter link needs to be set additionally for inputted noises. Therefore, accurate compensation currents are provided; the phase lag is small; and the anti-interference capability is high.