The invention discloses a semi-steel heat-preservation agent and the application of the semi-steel heat-preservation agent. With the overall weight of the heat-preservation agent serving as a standard, the heat-preservation agent comprises, by weight, 30%-50% of MgO, 4%-20% of Al2O3, 3%-20% of SiO2 and 20%-50% of C. According to the semi-steel heat-preservation agent, good heat-preservation effect on semi-steel is achieved, and particularly after the process of semi-steel tapping, the surface of the semi-steel is evenly coated with the heat-preservation agent, temperature reduction of the semi-steel in the process of transportation after the process of semi-steel tapping can be effectively reduced, energy can be saved, and the economic benefit can be remarkably improved.