The invention discloses a flocculant used in the process of removing impurities of steviol glycosides. The raw materials include: 10-20 parts of modified chitosan, 15-25 parts of modified lignin, and 30-40 parts of polyferric sulfate , 25-35 parts of aluminum citrate, 10-15 parts of magnesium sulfate, 20-30 parts of polysilicate iron, 3-5 parts of porous perlite powder, 4-6 parts of zeolite powder; wherein the preparation method of modified chitosan For: carboxymethyl chitosan is added in aqueous sodium hydroxide solution, heat up, after insulation, dropwise add 3-chloro-2-hydroxypropyltrimethylammonium chloride aqueous solution, then continue insulation, then adjust to neutrality to obtain The first material: add the first material to ethanol, filter, and dry to obtain the second material; add the second material to distilled water and stir evenly, then heat in a water bath, then add acrylamide and initiator dropwise in sequence, and keep warm to obtain the third material ; Add the third material to acetone, filter, and dry to obtain modified chitosan.