The utility model discloses a direct current supply device capable of obtaining electric energy from cable in service, aiming at solving the power supply problem of field cable on-line monitoring equipment. The direct current supply device has the technical scheme that the device consists of an electricity-taking mutual inductor, a rectifier bridge, a filter capacitor, a voltage-stabilizing circuit and a storage battery; wherein the electricity-taking mutual inductor is a current transformer, an annular magnetic choke is sleeved on a one-phase high-voltage cable, the input end of the rectifier bridge is connected with the output end of the electricity-taking mutual inductor, the direct voltage of the output end is connected with the input end of the voltage-stabilizing circuit after being filtered by the filter capacitor, the output ends of the voltage-stabilizing circuit and the storage battery respectively supply power to the monitoring equipment by passing through an isolating diode, and a charging resistor is connected on the isolating diode at the output end of the storage battery. The direct current supply device has a simple structure and low cost, has large voltage stabilization range to provide continuous and stable electric energy in the condition of large cable current changing span and is especially suitable for supplying power for the field cable on-line monitoring equipment.