The invention relates to an underwater constant-current constant-voltage conversion device suitable for an underwater constant-current power supply system, and the device comprises a main power conversion module, an analog load module, a communication control module, and an overvoltage overcurrent protection module. The main power conversion module is used for converting high-voltage DC constant-current input into low-voltage constant-voltage output, and the analog load module is used for guaranteeing the constant-power output of the underwater constant-current constant-voltage conversion device, and is in parallel connection with the main power conversion module. When the main power conversion module has an output fault, the analog load module is controlled to achieve the constant-power output. The communication control module is used by a shore base station for the remote control of the underwater constant-current constant-voltage conversion device, wherein the remote control comprises the adjustment of output power and the monitoring of underwater power supply state information. The overvoltage overcurrent protection module is used for avoiding the instantaneous overvoltage, surge current impact, earth induction current impact and reverse surge impact, which are caused by the short circuit of a marine cable in bypass power supply.