The invention mainly belongs to the technical field of steel production and particularly relates to a method for heating molten steel. According to the method, a microwave generator is adopted to generate microwaves, and the molten steel is heated by the microwaves. The temperature raising speed of the molten steel is controlled by adjusting the emission power of the microwave generator. When the temperature of the molten steel is raised to target temperature, the temperature of the molten steel can be preserved by reducing the emission power of the microwave generator. After temperature preservation ends, the microwave generator is shut down to stop heating. Microwave heating is used in the method, and compared with other molten steel heating modes, the method has the advantages of being high in heating speed, adjustable in heating speed, low in cost, capable of reducing energy consumption and the like.