The electromagnetic noise of permanent magnet synchronous motor (PMSM) has emerged as a critical research focus due to the increasing stealth requirements for underwater vehicles. This study investigates a 90 Nm motor with a maximum noise 95.9 dB, which exceeds customer specifications. First, the modal analysis of the motor was conducted using Ansys Maxwell to identify its vibration modes and natural frequencies; Second, the rotor structure was redesigned to reduce the magnitude of radial electromagnetic forces at operational points; Finally, a prototype was fabricated based on the analytical results, and noise testing was performed. Experimental results demonstrate a 3.8 dB reduction in maximum noise at the target operational point. This methodology provides practical insights for motor noise optimization.