The invention discloses a method for preparing a cation ester-based type soap-free emulsion polymer. According to the method, a cation type and an ester-based monomer are adopted as raw materials, a soap-free emulsion polymerization method is implemented, meanwhile, a pH value adjusting agent is adopted to control the pH value of a reaction environment within 8-8.5, a soap-free cation hydrophilicmonomer, a soap-free ester-based hydrophobic monomer and an initiator a are enabled to react to synthesize a soap-free polymer, and the soap-free polymer is further reacted with other raw materials tosynthesize a polymer emulsion. The preparation method has the advantages of being rapid in polymerization speed, small in reaction step, small in environment pollution and mild in reaction condition,the synthesized product is good in permeation resistance, good in peeling effect and excellent in waterproof effect and weather resistance, no expensive emulsifier is added in the synthesis process,and thus the production cost is also greatly reduced.