The invention discloses a constant temperature control method for a tundish electromagnetic heating power supply. In allusion to the tundish electromagnetic heating power supply adopting a pre-stage three-phase rectification and rear-stage full-bridge cascade structure, a three-phase current input mathematical model and a single-phase current output mathematical model are respectively built, a power-current double-loop control mode is adopted, and closed-loop control for the single-phase load power and system input-output current is realized. Meanwhile, in order to suppress three-phase input harmonic current generated by a variable-frequency load, a harmonic suppression controller is added to input current closed-loop control, and the electric energy quality at the input side is improved.Feedfoward control for the temperature of molten steel is added to the system, thereby realizing accurate tracking for output current of the electromagnetic heating power supply, realizing constant temperature control for the molten steel, and improving continuous casting yield and production efficiency of the steel.