The invention provides a method for extracting magnesium by means of high-pressure carbonization.The method includes steps of uniformly mixing water with raw materials with magnesium to obtain reaction slurry; carrying out carbonization reaction on the reaction slurry and carbon dioxide gas under the condition of gauge pressures of 1-5 MPa to obtain serous fluid; keeping the reaction pressures and separating solid from fluid in the serous fluid to obtain fluid with heavy magnesium; reducing the pressures and decomposing the fluid with the heavy magnesium to obtain decomposed serous slurry; separating solid from fluid in the decomposed serous fluid to obtain solid and drying the solid to obtain magnesium-rich products.The method has the advantages that the method can be applied to utilizing magnesium-rich minerals such as magnesite and magnesium-rich solid waste such as boric sludge, the carbonization magnesium extraction efficiency and the calcium and magnesium separation efficiency can be improved, and energy consumption and the economic cost can be reduced to a great extent; as compared with existing processes for extracting magnesium, the method is mild in operation temperature, little in variation, low in energy consumption and heat exchange difficulty and easy to implement, required equipment is small, and the like.