The invention relates to an ocean drag coaxial cable and photoelectric composite cable double-mode transmission device and method. The device comprises an overwater unit and an underwater unit. The power supply and communication of the overwater unit and the underwater unit can be carried out through a coaxial cable or a photoelectric composite cable. The overwater unit comprises a first modem, a first hybrid transmission module, a first 100M fiber transceiver, a first switch, a coaxial joint, a fiber joint, an aerial socket, a network socket and a power supply module. The underwater unit comprises a second modem, a second hybrid transmission module, a second 100M fiber transceiver, a second switch, a first watertight socket, a second watertight socket, a photoelectric composite cable watertight socket, and a power conversion module. According to the device and the method, the power supply and communication of the overwater unit and the underwater unit can be realized, underwater equipment is suitable for coaxial cable and photoelectric composite cable double-mode working, the application scene of the underwater equipment is expanded, the switching of the coaxial cable and the photoelectric composite cable is reduced, and the working efficiency is improved.