A cationic starch graft copolymer is prepared by polymerization of monomers a) to d) a) 20-80% by weight of acrylamide, methacrylamide or mixtures thereof, b) 3-20% by weight of at least one basic or cationic vinyl monomer; c) 0.005-1.5% by weight of at least one bifunctional or higher-functional crosslinking agent, d) 0-10% by weight of at least one nonionic or anionic vinyl monomer which differs from the monomers mentioned under a) to c) in the presence of a grafting base e) e) 15˜70% by weight of at least one starch or one starch derivative, the sum of a) to e) being 100% by weight.