To achieve contactless, all-weather power supply and data transmission for sensors on cable-tethered underwater platforms, an underwater single wire power and data parallel transmission system (USWPDT) is proposed in this paper, utilizing the mooring cable and open seawater as the transmission medium. The system model is analyzed, the underwater compensation structure is designed, and a method for suppressing crosstalk is proposed. Frequency division multiplexing couples the 2FSK modulated signal with the power waveform, achieving a 10 kb/s data transmission rate. The calculation and simulation results show that the load receiving power, efficiency and data demodulation are not greatly affected by the change of the underwater coupler position, which verifies the effectiveness of the proposed method.