[Objective] High-power permanent magnet direct-drive motors for port transmissions are usually designed with high voltage ratings. However, this solution poses significant challenges in terms of electromagnetic design, manufacturing precision and maintenance reliability. For variable-speed drive systems, relying on high-cost power electronics for frequency conversion greatly increases the production cost. To address these problems, a low-voltage multi-branch structure design for permanent magnet direct-drive motors is proposed in this paper. [Methods] Firstly, according to the design requirements of high-power permanent magnet direct-drive motor for port transmissions, the electromagnetic scheme of the motor was designed to use multiple low-voltage inverters to supply power, and the total current of the whole motor was shared by each branch circuit. Then, the stator winding of the motor was designed with multi-branch structure, and the coupling between different branch windings and the electromagnetic performance of the motor when different branches were put into operation were analyzed by finite element simulation to theoretically verify the feasibility of the multi-branch motor. When different branches were put into operation, it increased the unevenness of the air-gap magnetic field and affected the stability of the motor. Finally, the cogging torque of the motor was optimized by opening an auxiliary slot in the rotor. [Results] Simulation results showed that the low-voltage multi-branch structure scheme, which effectively solved the problem of high current in the low-voltage scheme, put different branches into operation according to the load demand and improved the overall efficiency of the motor. When a certain inverter failed, the reliability of the motor drive system was greatly improved by controlling other inverters and increasing the current so that the motor output the rated torque. The peak-to-peak value of cogging torque of the un-slotted motor was 2 419.5 N·m, and after opening the rotor auxiliary slot optimization, the peak-to-peak value of cogging torque was reduced to 1 136.7 N·m, a reduction of 53.02%. [Conclusion] The low-voltage multi-branch structure optimization scheme proposed in this paper effectively solves the high cost and high current problems of high-power permanent magnet direct-drive motors for port transmission. The scheme can decide the number of branches to be put into operation according to the load requirements, which improves the efficiency of the motor and provides a certain reference for the design of high-power motors.