Abstract: Addressing the issues of significant voltage drop, high power loss, and response delay in the power supply process for long-distance deep-sea operation-level ROVs, this paper proposes and designs an umbilical cable long-distance power supply system based on a 400Hz AC power source. Firstly, the high-voltage AC power supply structure of ROVs and the frequency domain characteristics and system adaptability of the 400Hz power source are analyzed, and a power supply link topology and key component configuration model are constructed. Secondly, a modeling method for cable impedance and voltage drop is established, and system voltage drop and power consumption performance analysis are completed based on the Simulink simulation platform. Finally, a test platform is integrated with a frequency converter, a step-up transformer, and a 3000m composite umbilical cable for engineering verification. The results show that the designed system can control the voltage drop within ±1%, with three-phase power loss less than 35W, exhibiting good voltage stability and dynamic response capability. This research provides an engineering basis for the lightweight and efficient power supply scheme of deep-sea ROVs.