The invention relates to a coating additive. The coating additive is prepared from the following components in percentage by weight: 12-18% of aluminium-magnesium silicate, 2-5% of white carbon black,0.5-0.8% of PVB, 4-7% of phenolic resin powder, 3-5% of maleated rosin powder, 4-8% of attapulgite, 1-2% of sodium hexametaphosphate, 4-8% of organobentonite, 10-15% of spodumene, 15-20% of mica powder, 15-20% of anhydrous sodium sulphate and 6-10% of a dispersing agent. A preparation method of the coating additive comprises the following steps that all the components are mixed according to the weight percentage, stirring is conducted for 10-15 minutes by a spiral rod stirrer, and after even stirring, the powdery coating additive is obtained; and during using, aggregate and a solvent only need to be added into the coating additive, stirring is conducted for at least 20 min by a high-speed dispersion machine, and thus the coating additive can be applied. Compared with the prior art, the coating additive is easy to produce and convenient to package and transport, and all aspects of performance of casting coating made of the coating additive is superior.