The invention belongs to the technical field of building materials, particularly relates to a steel slag compound basic magnesium cement building material, and further discloses a preparation method of the material. According to the steel slag compound basic magnesium cement building material, low-grade light calcined magnesia, steel slag and magnesium sulfate after industrial waste sulfuric acidtreatment serve as raw materials, and high-performance building material products are prepared by means of carbonization treatment. The maximum strength of the steel slag compound basic magnesium cement building material can reach 80MPa or more, the softening coefficient of the material can reach 0.92 or more, and the material has high strength, long service life and good water resistance, can beused in a humid environment and even underwater and overcomes the shortcoming that basic magnesium cement cannot be used in the humid environment.