The cationic styrene-acrylic latex used for redispersible polymer powder was prepared by the method of seeded emulsion polymerization by using cetrimonium bromide(CTAB) as emulsifier, azobisisobutryamide chloride(AIBA) as initiator, and [2-(methacryloyloxy)ethyl]trimethyl-ammonium chloride(DMC) as hydrophilic cationic monomer and acrylamide(AM) as functional monomer while the mass ratio of DMC to AM was 1:1. The effects of polymerization temperature, the amount of emulsifier, initiator, seed monomer, and cationic monomer on polymerization reaction system and properties of latex and redispersible latex powder were investigated. The optional formulation and process condition is that the polymerization temperature is 80±2℃, the amount of CTAB, AIBA, seed monomer and DMC are 2%, 0.53%, 10% and 2% based on total monomers respectively. Then an uniform and stable latex with small particle size and narrow particle size distribution could be obtained. Redispersible latex powder made from this latex has low water content and small average particle size, and also has good redispersibility.