The invention relates to a temperature control device and in particular relates to a constant-temperature device in a direct-current chamber of a power system. The constant-temperature device comprises a cast-iron radiator (1), a temperature controller (2) and a relay (3), wherein an electric heating tube (4) is mounted in a water inlet in the bottom of the cast-iron radiator (1) in a sealing manner; an input end of the temperature controller (2) is connected with a temperature sensor (6) which is mounted on the side surface of the cast-iron radiator (1); an output end of the temperature controller (6) is connected with a coil of the relay (3); a contact of the relay (3) is connected in series in a circuit of the electric heating tube (4); the electric heating tube (4) is powered by a 220V power supply. The constant-temperature device is reasonable in design; by using waste or obsolete cast-iron radiator, the constant-temperature device has the main advantages of simplicity, practicability, large heat radiating area, long heat supply and heat preservation time, durability and economy.