Disclosed is a method of preparing a biocide having improved durability of its biocidal activity as well as disinfection efficiency at an initial stage, comprising the steps of: (a) preparing stabilized alkali or alkaline earth metal hypochlorite having a pH at least 11 by mixing a chlorine oxidant including alkali or alkaline earth metal hypochlorite with a stabilizer in an alkali solution; (b) preparing a bromide ion source; and (c) adding the bromide ion source prepared in step (b) into the stabilized alkali or alkaline earth metal hypochlorite prepared in step (a). Also, a method of controlling the growth of microorganisms using a biocide prepared by the method of the present invention is disclosed.