The invention provides an aluminum-calcium-magnesium-cerium composite alloy used for strong desulfurization, final desoxidation, and microalloying quenching and tempering of liquid steel, belonging to the technical field of ferroalloy used for steelmaking. Based on aluminum deoxidization, the aluminum-calcium-magnesium-cerium composite alloy is added with alkaline-earth metal calcium and magnesium to conduct the strong desulphurization and the final desoxidation on the liquid steel so as to reduce inclusions in steel, and adopts rare earth-cerium and alloy elements such as titanium, niobium, vanadium and the like to conduct quenching and tempering, and microalloying on the liquid steel so as to change the morphology of the inclusions, lead the steel to be solidified, crystallized and refined, and optimize the steel performance. The aluminum-calcium-magnesium-cerium composite alloy contains the following chemical compositions (mass percent): 10-60% of aluminum, 10-30% of calcium, 10-30% of magnesium, 0.5-10% of cerium, less than or equal to 1% of impurities (carbon + silicon + phosphorus + sulphur), and the balance of ferrum; and for the purpose of achieving different microalloying, the composite alloy also can contain one or more microalloying elements of titanium, niobium, molybdenum, vanadium, tungsten and boron, and the weight percentage content of the microalloying elements is 0.1-10%. In the invention, the final desoxidation, the strong desulfurization, the microalloying and the quenching and tempering of the liquid steel is completed in one step, thus improving the refining efficiency and effect, and greatly improving the steel quality.