The invention discloses a method for purifying capsular polysaccharide, which comprises the following steps of: centrifuging the capsular bacterium fermentation liquid, collecting the supernatant liquid, carrying out ultrafiltration concentration and ethanol precipitation, and collecting the supernatant liquid; carrying out ethanol precipitation, collecting precipitates, and redissolving injection water; adding protease for enzymolysis, and carrying out ultrafiltration on protein hydrolysates; adding hexadecyl trimethyl ammonium bromide and NaCl of which the mass percent final concentration is 0.2-0.3%, centrifuging, and collecting the supernatant liquid; adding hexadecyl trimethyl ammonium bromide and NaCl of which the mass percent final concentration is 0.1-0.15%, centrifuging, collecting precipitates, and dissolving the precipitates; carrying out ethanol precipitation, collecting precipitates, and redissolving injection water; and carrying out ultrafiltration and membrane filtration. Through the purification method disclosed by the invention, the recovery rate of the capsular polysaccharide is high, the contents of protein and nucleic acid are low, the step of utilizing toxic organic reagents in the traditional purification method is omitted, the purification steps are simplified, and the time and cost are saved.