The invention discloses a circulating water electrolytic descaling device and a descaling method thereof, and belongs to the technical field of environment protection. The device comprises a reaction chamber, a water inlet, a water inlet valve, a water outlet, a water outlet valve, a drain outlet, a drain valve, an anode, a control system, an end plate and a bracket. The reaction chamber therein is provided with a test anode, the reaction chamber is made of metal materials, and the inner wall thereof is used as the cathode of an electrolytic reaction and connected with a cathode wiring terminal; the anode and the test electrode pass through the end plate and are and fixed on the end plate, the test electrode approaches the inner wall of the reaction chamber, the outer wall of the reaction chamber is coated with an epoxy resin layer and provided with an ultrasonic cleaner, the bottom of the reaction chamber is spherical or tapered, and the inner surface thereof is coated with the epoxy resin layer. The descaling method comprises an electrolytic descaling process and a cleaning descaling process. The invention uses ultrasonic waves for automatically descaling and cleaning, has the functions of automatically sterilizing, removing alage and softening water quality after descaling, has multiple functions, compact, firm and durable structure, and is suitable for sterilization, algae removal, descaling and corrosion resistance of circulating cooling water.