The invention discloses a concrete additive taking silico-calcium oxide as a main component and a preparation method. The concrete additive consists of compounded industrial waste residue powder and mountain flour, and comprises the following components in percentage by mass: 45-55% of calcium oxide, 20-35% of silicon dioxide, 1-3% of aluminum oxide, 1-2% of magnesium oxide and the balance of impurities, a silicate vitreous body and an aluminate vitreous body are taken as main mineral components, the fineness of the concrete additive is 45 mu m, and the weight of screen residues is 12-25%. By adopting the preparation method, high-temperature calcinations is not needed, and by only controlling the temperature, the grinding fineness and the mixing ratio of mechanical grinding, a composite activated material which can have chemical reaction with main hydration products of cement in an alkali environment can be produced. The composite activated material can effectively stabilize a cement hydration product system, improve the density and the mechanical strength of the cement hydration product system, and reduce the temperature rise of a cement hydration system, and can be applied to concrete construction engineering of industries such as water conservancy and hydropower.