The invention discloses ultralow-alkalinity high-vitrification covering slag for bloom medium-high carbon steel and a preparation method thereof, and belongs to the technical field of steelmaking metallurgy auxiliary materials. The ultralow-alkalinity high-vitrification covering slag for the bloom medium-high carbon steel is prepared from the following raw materials of, by weight, 15-35 parts of wollastonite, 15-30 parts of power plant molten coal cinder waste, 2-13 parts of quartz, 6-13 parts of albite, 2-8 parts of sodium fluoride, 1-5 parts of sodium carbonate, 6-10 parts of fluorite, 2-5 parts of magnesia powder, 1-6 parts of 390 medium-carbon graphite, 2-6 parts of high-carbon amorphous graphite, 1.5-3 parts of semi-reinforcing natural gas carbon black and 5-8 parts of sodium silicate. The ultra-low alkalinity is adopted, the vitrification rate in the slag solidification process is controlled to be high, and then the blank shell thickness and the lubricating effect in the medium-high carbon continuous casting process are guaranteed; the high viscosity is adopted, under the condition that a casting blank has certain shrinkage, molten slag is evenly guided in, solid slag film is uniform, and it is guaranteed that the surface of the casting blank is smooth; slightly high melting point and viscosity are adopted, so that the use amount of flux is reduced, the slag surface tension is improved, and the slag entrapment probability is reduced; and compound carbon blending is adopted, and then the problem of accidental carbon pickup of the casting blank is relieved by adjusting the carbon blending.