To provide a method for improving flocculation by making the polymerization degree of cationic polymer higher.

The method for the preparation of an aqueous cationic polymer solution includes steps of: (a) polymerizing substantially all of a monomer component having at least one monomer having a cationic functional group by reaction initiated from a radical initiator to form a base cationic polymer solution; and (b) contacting the base cationic polymer solution with an additional radical initiator to cause multiple cationic polymer groups to form interconnecting bond so that the base cationic polymer solution forms an aqueous solution containing a multi-crosslinked cationic polymer having a higher molecular weight than the base cationic polymer.