The present invention provides a method for preparing a water soluble polymer, which comprises: a first step of polymerizing: (a) 60 to 99.5 mol % of at least one selected from acrylamide and methacrylamide, (b) 0.5 to 20 mol % of at least one selected from α,β-unsaturated monocarboxylic acid, α,β-unsaturated dicarboxylic acid and salts thereof, and (c) 1 to 20 mol % of at least one selected from a water soluble cationic monomer and a salt thereof, (d) 0.01 to 1 mol % of a chain transfer agent, and (e) 0.005 to 5 mol % of a crosslinkable monomer, in the presence of a persulfate-based catalyst to obtain a prepolymer; and a second step of further adding the persulfate-based catalyst to the prepolymer obtained in the first step and the residual monomer, followed by the polymerization and termination of the polymerization. Consequently, it is made possible to suppress a rapid increase in viscosity during the polymerization thereby to obtain a water soluble branched polymer having high molecular weight and low viscosity.