The invention discloses a variable-speed driving device of a cable water damping test system. The variable-speed driving device comprises a stander, a power unit, a transmission mechanism and a speed measurement unit, wherein the power unit comprises a variable-frequency motor and a frequency converter; a rail is arranged above the stander; the transmission mechanism adopts a chain wheel/chain transmission mechanism; the speed measurement unit comprises a wheel rope transmission structure, a photoelectric encoder, a data acquisition system circuit board and a computer; the wheel rope transmission structure comprises a plurality of transmission pulleys arranged on the stander; the input end of the photoelectric encoder consists of two rotating wheels; the two rotating wheels and one transmission pulley of the wheel rope transmission structure form a middle wheel rope transmission structure; the photoelectric encoder is connected with the data acquisition system circuit board; and the data acquisition system circuit board is connected with a computer. The cable water damping test system with a power and speed regulation device can simulate a mechanical model of a bond cable subjected to an ocean current at different flow speeds, so that measurement of a damping force on the bond cable in a water environment can be realized.