To reduce the cogging torque of interior permanent magnet synchronous motors, this paper investigates the method of adding auxiliary slots to the rotor. Firstly, the mechanism of cogging torque generation is theoretically analyzed. The addition of auxiliary slots to the rotor will alter the magnetic field distribution. According to the theory of the auxiliary slot function, a scheme of double-semicircular auxiliary slots of the rotor is proposed to reduce the cogging torque. A finite element analysis (FEA) model of a 6-pole 36-slot V-shaped interior permanent magnet synchronous motor is developed. Double-semicircular auxiliary slots are added to the rotor surface along the d-axis. The influence of the slot’s size and shape on cogging torque is analyzed, and the variation patterns of cogging torque to the geometry of the auxiliary slots are summarized. The results demonstrate that the cogging torque can be effectively reduced and the motor’s other performance parameters are largely preserved, by optimizing the design of rotor auxiliary slots. The double-semicircular auxiliary slots design minimizes air-gap flux density distortion, suppresses cogging torque harmonics, and optimally reduces cogging torque.