Isatis root refined polysaccharide capable of improving immunity as well as protein removal and purification method and application thereof

The invention provides radix isatidis refined polysaccharide capable of improving immunity and a preparation method of the radix isatidis refined polysaccharide. The radix isatidis refined polysaccharide comprises mannose, rhamnose, galacturonic acid, glucose, galactose and arabinose according to a molar ratio of 2.17: 0.96: 2.90: 83.25: 4.88: 5.84. The method comprises the following steps: performing water extraction on an isatis root polysaccharide extracting solution, adding trypsin to perform enzymolysis on protein molecules, and repeatedly freezing and thawing to remove protein and trypsin together. Protein impurities in the radix isatidis extracting solution are effectively removed through a trypsin-repeated freezing and thawing combined method, and the structure of radix isatidis polysaccharide is not affected. The purified isatis root polysaccharide provided by the invention can obviously increase the zebrafish immune cell density, increase macrophage proliferation, reduce the IL-1beta and IL-6 conten