The invention discloses a method for purifying ulinastatin based on a cation exchange resin. The method comprises: S1, weighing 1L of fresh urine, putting the fresh urine in a beaker, adding 12-18 g of onychin to the beaker, performing stirring for 80 min, measuring the acid-base value of the stirred urine via a pH meter, controlling the pH of the stirred urine in a range of 6.2-7.3, and performing filtration closure on the urine with the pH in a range of 6.2-7.3 via an ultrafiltration membrane to obtain pre-treated urine; S2, adding the urine prepared in the S1 from the top end of an ion exchange column, controlling the exchange flow rate of the ion exchange column in a range of 0.001-0.005 m/s, and using a beaker to get exchanged ulinastatin at the lower end of the ion exchange column for later use; and S3, drying ulinastatin prepared in the S2 at 60 DEG C in a vacuum drying oven for 6-9 h, and finally, freezing the obtained ulinastatin in a refrigerator for 1-2 h to obtain a pure ulinastatin sample. The method is simple and convenient to operate, the purity of purified ulinastatin can reach no less than 99.75%, and has the specific activity at least being 4880 U/mg egg white, and the yield is no less than 78.5%.