An additive for increasing an early activity index of nickel slag and a preparation method thereof, belonging to the field of additive technologies, are provided. The preparation method includes: successively adding maleic anhydride and triethanolamine to a reactor; setting the heating temperature to 50° C. for reaction, where a large amount of heat is released during the reaction; when the reaction temperature decreases to 60° C. after heat is released in the reaction, allowing triethanolamine maleate to react with a solution of bromine in carbon tetrachloride; adding water to the mixture, where the weight percentage content of the added water is 60%; separating and removing carbon tetrachloride from water; and conducting uniform stirring to obtain the additive. A molar ratio of the maleic anhydride, the triethanolamine, and bromine is (0.2-1):1:(0.2-1), and a molar ratio of the maleic anhydride to the bromine is 1:1.