The invention discloses a biological slime inhibitor. The biological slime inhibitor is characterized by comprising the following components by weight percent: 5-30% of poly-hexylidene triamine guanidinium chloride, 1-10% of isothiazolinone, 1-10% of organic bromide, 5-40% of methanol solvent, and the balance of water, wherein the weight-average molecular weight of the poly-hexylidene triamine guanidinium chloride is Mw=10000-20000; the molecular weight distribution D is 1.0-1.1; the isothiazolinone is a mixture of 5-chloro-2-methyl-4-isothiazolin-3-ketone and 2-methyl-4-isothiazolin-3-ketone, of which the mass ratio is (2.5-4):1; the organic bromide is one or more of 2,2-dibromo-3-nitrogen propionamide, 2-brome-2-nitro-1,3-propanediol and 2,2-dibromo-2-nitroethanol. In addition, the invention also provides a preparation method of the biological slime inhibitor. The biological slime inhibitor disclosed by the invention has an extremely efficient sterilization effect and excellent alga-killing property, and has the characteristics of being long in bacteriostatic time, free of drug resistance, free of a bubble, safe to use and the like.