The invention discloses a viscosity stabilizer for an oil-displacing polymer solution and a preparation method of the viscosity stabilizer. The stabilizer composition includes a solvent and additives;the additives are at least two selected from the group consisting of an antioxidant, an anti-salt agent and a bactericide; the antioxidant is at least one selected from the group consisting of thioethanol, thiourea, thiodipropionic acid, sodium thiosulfate, potassium iodide, 2-hydroxyethylamine and acetone; the anti-salt agent is at least one selected from the group consisting of ethylenediaminetetraacetic acid or a salt thereof, 1-hydroxyethylidene-1,1-diphosphonic acid or a salt thereof, polyacrylic acid and sodium citrate; and the bactericide is at least one selected from the group consisting of N,N-dimethyldithiocarbamic acid sodium salt, methylene dithiocyanate, 2,2-dibromo-3-cyanoacetamide, 2-nitro-2-bromo-1,3-propanediol, glutaraldehyde and isothiazolinone. The stabilizer solves the problem of large viscosity loss of the polymer solution during polymer displacement.