The present invention discloses a biological slime stripping agent for a water treatment. The biological slime stripping agent is characterized by comprising, by weight, 5-30% of alkyl polyhexamethylene guanidine hydrochloride, 1-10% of isothiazolinone, 1-10% of organic bromine, 5-40% of a solvent, and the balance of water, wherein the alkyl polyhexamethylene guanidine hydrochloride structure is represented by a formula (I), m is 20-90, n is 9-17, the weight average molecular weight Mw is 12000-20000, the molecular weight distribution D is 1.0-1.2, the isothiazolinone is a mixture of 5-chloro-2-methyl-4-isothiazolin-3-one and 2-methyl-4-isothiazolin-3-one, the organic bromine is one or a plurality of materials selected from 2,2-dibromo-3-nitrilopropionamide, 2-bromo-2-nitro-1,3-propylene glycol, and 2,2-dibromo-2-nitroethanol, and the solvent is an alcohol solvent.