The invention relates to the technical field of metallurgy, in particular to a molten steel heating method of continuous casting tundish. Firstly, a long nozzle is arranged between a ladle and a tundish, then an electromagnetic sensor is arranged around the long nozzle, the power of the electromagnetic sensor is automatically controlled by determining the temperature at the nozzle, the molten steel which flows through the long nozzle is heated, and the molten steel temperature of the tundish is adjusted; the electromagnetic sensor is in a split type, and is closed and separated through an opposite-pull method or a door spindle method; when the long nozzle is changed or detached in the late period of casting, the electromagnetic sensor is separated through the opposite-pull method or the door spindle method and moved to a certain station, so that the operation of relevant continuous casting processes is conveniently carried out; the power of the automatic control electromagnetic sensor is 700kW to 1200kW; and the molten steel temperature of the tundish is regulated to be +/-5DEG C range of the target temperature. According to the method, the heating efficiency is high, the control performance is good, the maintenance is convenient, the molten steel is not polluted, the casting at constant temperature and constant speed is realized, and the quality of a casting blank is improved.