This paper presents a method for reducing the cogging torque and ripple torque of interior permanent magnet synchronous motor (IPMSM) based on the magnetic field harmonic offset method. This method establishes the internal correlation between cogging torque harmonics and ripple torque harmonics. The suppression or cancellation of magnetic field harmonics in the rotor pole is utilized as transmission link to simultaneously weaken or eliminate lower order harmonics of cogging torque and ripple torque, which can improve operating quality of the IPMSM and obtain an acceptable total average torque. A mathematical and physical model of harmonic offset method for cogging torque is established, the distribution characteristics of permeability harmonics and field harmonics that affect cogging torque are analyzed, the analytical expression for the electromagnetic torque of the IPMSM including reluctance torque is derived, and the collaborative suppression mechanism of cogging torque and ripple torque, as well as common solutions, are studied. Finally, the suppression law of cogging torque and operating ripple torque is verified by the finite element simulation, and the compromise selection principle of permanent magnetic pole is summarized. Due to the absence of the average torque of motor in the offset method, the effects of effective pole arc of the combined rotor on the torque ripple and torque-speed characteristic curve of the IPMSM are compared and evaluated.