To reduce the cost of the permanent magnet (PM) in the external rotor permanent magnet synchronous motor (ERPMSM) when used as a drive motor for a belt conveyor, and to improve the motor performance, an optimization design method based on Response Surface Methodology (RSM) and Improved Multi-objective Whale Optimization Algorithm (IMOWOA) is proposed. Based on the basic structure of the motor, optimization parameters such as the permanent magnet size, air gap length, and slot width are considered, while optimization objectives include the permanent magnet cost, output torque, and torque ripple. Through parameter sensitivity analysis, significant parameters are selected. A sample space is established using RSM combined with finite element simulation, and the functional relationship between optimization objectives and parameters is fitted. The IMOWOA algorithm is used for optimization. Finally, the results of the optimized and non-optimized schemes are compared. The proposed multi-objective optimization algorithm is accurate, reliable, and demonstrates better convergence and diversity, enabling the reduction of permanent magnet costs while optimizing motor performance.