The traditional direct torque control(DTC)of interior permanent magnet synchronous motor(IPMSM)suffers from two major problems of variable switching frequency and large torque ripples. In this paper, a constant switching frequency-based DTC algorithm for IPMSM drives is proposed. The method consists of only one PI regulator and one triangular-wave carrier. By comparing the output of the PI torque regulator with the fixed frequency carrier signal, the switching frequency of the inverter is constant, which effectively reduces the torque ripple in the motor operation and improves the current distortion; and a detailed mathematical modeling process and a small signal stability analysis are provided for the proposed torque regulator. Simulation results show that the proposed algorithm can effectively improve the steady-state operating performance of the direct-torque control system of the interior permanent magnet synchronous motor, and the switching frequency of the system remains approximately constant under the premise of maintaining a fast dynamic response.