The invention relates to an external cold water descaling device for a direct-current converter valve cooling system and a method. The external cold water descaling device comprises a descaling tank, a direct-current power source, an electrode, an ultrasonic generator and a scale residue collection tank, a cathode and an anode of the electrode are corresponding connected with a cathode and an anode of the direct-current power source, the electrode is fixedly mounted in the descaling tank, the ultrasonic generator is mounted on the descaling tank, the scale residue collection tank is mounted at the bottom of the descaling tank, and the bottom of the descaling tank is open. The method includes: fixedly mounting the electrode with the descaling tank, and applying low voltage on the electrode; enabling calcium and magnesium ions of spraying water to be in chemical reaction at the cathode of the electrode to generate fouling; applying vibration of certain frequency on the electrode, and adjusting vibration frequency of the ultrasonic generator to vibration frequency identical when the fouling is adhered on the electrode to enable the fouling to shed into the scale residue collection tank. Through the descaling device and the method, scaling ions in spraying water can be removed, and the scaling problem of a cooling tower in a convertor station is solved.