Aiming at the practical engineering problem of high torque ripple of low speed high torque motorized spindle motor, the performance of different slot number combination of the motor is analyzed and compared to accurately select the motor scheme with better performance. The convergence speed of transient field in finite element simulation of induction motor is slow, and a long simulation period is needed when comparing multiple schemes. Aiming at these problems, the joint simulation of time harmonic field and transient field is proposed to realize the rapid optimization design of the motor. Two-dimensional electromagnetic finite element simulation is carried out for a 12-pole, 7.5 kW motorized spindle induction motor with three different slot number combinations of 3, 2.5 and 2 slots per pole per phase, and the electromagnetic properties of the motor under the rated conditions such as iron loss, winding copper loss, rotor aluminum loss, efficiency, power factor and torque ripple are compared. The results show that increasing the number of rotor bars can lead to better electromagnetic performances, especially a low level of torque ripple. Finally, a prototype with 90 stator slots and 106 rotor slots is made, and the performance of the prototype, as well as the accuracy of the joint simulation of time harmonic field and transient field, is verified by experiments.