The invention provides a slag vertical mill grinding assisting activating agent and its compounding method. The slag vertical mill grinding assisting activating agent is composed of, by weight percent: 10-12% of industrial salt, 3-6% of urea, 8-12% of triethanolamine, 10-15% of glycerol, 8-12% of an aluminum sulfate solution, 3-6% of beet molasses, 5-8% of polycarboxylic acid, and the balance a solvent. The method consists of: adding the industrial salt, the urea, the triethanolamine, the glycerol, the aluminum sulfate solution, the beet molasses, and the polycarboxylic acid into the solvent in order, fully dissolving each raw material when it is added, and conducting uniform stirring, then adding another raw material till all the raw materials are added into the solvent for dissolving, and mixing the mixture uniformly, thus obtaining the slag vertical mill grinding assisting activating agent, which can play a grinding assisting role and an activating role. By employing a compatible mode of chemical excitation and slag powder particle size distribution improvement, the slag vertical mill grinding assisting activating agent provided in the invention has an advantage complementary effect, not only adapts to low activity acidic slag, but also adapts to neutral and alkaline slag, and is applicable to both vertical mills and ball mills.