The invention provides a method for electromagnetically heating a liquid. As a high-frequency coil is wound outside a strong magnetic material, the method has the advantages of quick temperature rise and high efficiency; a shielding water jacket and a waste steam heat utilization tank are arranged, so that the waste heat of a heater is made full use of and is accumulated; the heater belongs to an electrical product applied in a household, a factory, a school and the like; the periphery of an inner shell of a heating body is heated; surrounding waste heat is accumulated in a heat storage medium; and meanwhile, water is heated by using the waste heat, therefore, the heat source is made full use of, the heater manufactured by using the invention can be big or small, can be produced in a standardized, universalized, industrialized and serialized manner, and can be widely applied to occasions such as a hotel, a restaurant, a household, a factory and the like; energy is saved, and emission is reduced; moreover, the heater is safe and reliable and can be widely suitable for energy-saving methods for electrically heating the liquid.