The invention discloses a method for determining the molecular weight of hydrophobically-associating type polyacrylamide. According to the method, a composition of formamide, sodium chloride and water is selected as a solvent; the relationship between an intrinsic viscosity number and the molecular weight of the associating polymer is determined by adopting partially-hydrolyzed polyacrylamide series samples with the known molecular weights, and thus an experiment method for determining the molecular weight of the hydrophobically-associating type polyacrylamide is determined. The method has full theoretic support in the test principle; the determining method has the advantages of simple instrument equipment, convenience for operation, short measuring and data processing period and the like; and the determining method is suitable for hydrophobically-associating polymers with different molecular weights, such as HAWSP (Hydrophobically Associating Water Soluble Polymers) with different hydrophobic types and different hydrophobic monomer contents, and is broad in application prospect.