The water-cooled flow channel in the casing of air compressor motor for fuel cell was designed and analyzed in this paper. Firstly, motor-CAD was used for electromagnetic simulation to determine the heat sources and heating power. Based on the heating power, the coolant flow rate was determined, and the convective heat transfer coefficients of the air gap and the main contact areas of the motor were calculated. A fluid-solid coupling model of the spiral-flow-channel water-cooled motor was established in ANSYS-Fluent software. The temperature at the end-region winding was validated by measuring the internal temperature of the motor using a PT100 platinum resistance temperature detector. The temperature rises of the motor was evaluated for the original flow channel under normal temperature conditions (coolant temperature ≈75 ℃) and high-temperature limit conditions (coolant temperature ≈85 ℃). The results show that the original design could meet the normal motor operation under normal temperature conditions. Under high-temperature limit conditions, the original design caused local overheating in the motor's right end-region winding, exceeding the H-level insulation temperature rise limit of 125.00 K. An optimized flow channel design was proposed to reduce the rate of cross-sectional area increase, thereby slowing the decrease in fluid velocity. Compared with the original design, the optimized channel increased the flow velocity at the overheating region, enhancing the coolant-motor heat transfer. The simulations results indicate that under high-temperature limit conditions, the optimized channel had a smaller cross-section change rate and a slower velocity reduction. The inner wall temperature decreased from 356.50 K to 354.85 K, and the maximum end-winding hot spot temperature decreased from 433.09 K to 422.92 K. Compared with the original design, the optimized end-region winding exhibits a reduction of 10.17 K in the local hot spot temperature. Furthermore, the temperature rise is 124.92 K, which is below the specified limit of 125.00 K. This demonstrates a significant improvement in the cooling performance of the flow channel.