The invention relates to a chemically bonded cementitious material applicable to seawater mixing, and a preparation method and a use method thereof. The cementitious material provided by the invention is prepared from a mixture of 60-90% of a slag micropowder, 5-15% of sodium silicate, 2-10% of silica fume, 0.5-3% of copper oxide and 0.5-5% of calcium nitrite. The cementitious material can replace ordinary cement to prepare marine concrete; the cementitious material can be mixed by seawater in the preparation process, heat release quantity is small, and the eventually-prepared marine concrete has a compact microstructure; and the cementitious material can improve the concrete strength, and allows working performance of the prepared marine concrete to be promoted, wherein the cementitious material can significantly improve the steel bar corrosion resistance and the anti-penetrability performance of the concrete and reduce the chloride ion diffusion coefficient and the like, so that the prepared marine reinforced concrete is allowed to have a good ability to resist seawater erosion.