To provide a method for powdering a water-soluble polymer, which, when powdered, needs no classification step and causing no problems, e.g. scattering of fine particles, and, when redissolved to be used as a coagulant or the like, causes no problems, e.g. lowered viscosity.

This method for producing a slurry composition containing a water-soluble polymer polymerizes a monomer containing N-vinyl carboxylic acid amide shown by the formula CH2=CHNHCOR (R is hydrogen or an alkyl), hydrolyzes the polymer with an acid, and incorporates the hydrolysis product with a basic compound at a temperature T(°C) in the following range: -2.26W≤T≤-2.26W+89 (W is content (%) of volatiles the water-soluble polymer contains while the hydrolysis product is being incorporated with the basic compound). The method for producing the polymer composition removes the solvent from the slurry composition, to produce the powder containing the water-soluble polymer.