As for the problems of the difficulty of heat dissipation and uneven temperature distribution in the rotor area of large-scale permanent magnet synchronous traction motors for high-speed trains， a novel cooling structure is proposed， which involves adding axial rectangular air passages on the outer surface of the stator core of the water-cooled motor housing， and forms an internal and external dual-cycle cooling structure in conjunction with the air gap and rotor lightweight holes. The purpose is to investigate the impact low of reducing the temperature rise in the stator and winding areas and improving the uniformity of internal motor cooling. Firstly， simulations are conducted using the Ansoft Maxwell platform to obtain the losses of various components in the dual-cycle cooling structure under rated operating conditions. To better simulate the airflow in the air gap driven by the rotation of the rotor， the air gap is treated in layers， and a fluid-structure coupled finite element analysis method is used to study the airflow characteristics and temperature rise patterns inside the motor under both single and dual-cycle cooling structures. The results indicate that the internal circulation air-cooling structure significantly increases the airflow velocity inside the motor and markedly improves the average heat transfer coefficient on the surface. As a result， more heat in the rotor area is transferred to the relatively lower temperature stator area and housing， while reducing the heat transferred to the rotor， thereby reducing the temperature rise of the rotor and permanent magnets. Furthermore， the orthogonal analysis method is used to optimize the structural parameters of the rectangular ventilation holes， including the cross-sectional area， quantity， and aspect ratio. The temperature rise uniformity coefficient is used to evaluate the temperature rise of the motor， and the optimal solution results in a 12.1 K reduction in the maximum temperature rise when compared to the single-cycle cooling structure and a 16.54% improvement in the overall temperature rise uniformity of the motor.