A preparation method of trihydroxyethyl rutoside, characterized by that 7-monohydroxyethyl rutoside is first prepared by ethylation of rutin, purified thereafter, and then converted to trihydroxyethyl rutoside by hydroxyethylation, wherein: in the step of preparation of 7-monohydroxyethyl rutoside, the hydroxyl groups of rutin are protected by a hydroxyl-protecting agent before reaction with a hydroxyethylation agent in a solvent, wherein the hydroxyl-protecting agent is borax, the hydroxyethylation agent is ethylene oxide, the solvent is selected from water, methanol and ethanol, the reaction temperature is 30-50°C and the reaction time is 4-14 hours; in the step of purification of 7-monohydroxyethyl rutoside, a recrystallization method is used for purification, wherein the solvent for recrystallization is selected from water, methanol, ethanol, isopropanol, and a mixture thereof, and wherein the recrystallization method includes dissolution, crystallization and filtration; in the step of preparation of trihydroxyethyl rutoside from purified 7-monohydroxyethyl rutoside, a hydroxyethylation agent is added after dissolving or dispersing 7-monohydroxyethyl rutoside in a solvent, wherein the hydroxyethylation agent is ethylene oxide, the solvent is selected from water, methanol, ethanol, pyridine and a mixture thereof, a catalyst is selected from sodium hydroxide, potassium hydroxide and ammonium water, the reaction temperature is 50-80°C and the reaction time is 3-8 hours, and the product is passed through cation and anion resins, or is treated with macroporous resin after the reaction is complete; wherein said cationic resins are selected from strong acidic polystyrene cation exchange resins, such as 007X7, 001X12, 001X1 and 001X16, the anion resins are selected from strong basic polystyrene anion exchange resins, such as 201X4 and 201X7; wherein said macroporous resins are selected from non-polar polystyrene macroporous resins, such as D101, Shanshi101, and low polar macroporous resins, such as D A, DAB-8 and D301; in the step of purification of trihydroxyethyl rutoside, a recrystallization method is used for purification, wherein a solvent for recrystallization is selected from water, methanol, ethanol, isopropanol, and a mixture thereof, and wherein the recrystallization method includes dissolution, crystallization and filtration. in the step of preparation of 7-monohydroxyethyl rutoside, the hydroxyl groups of rutin are protected by a hydroxyl-protecting agent before reaction with a hydroxyethylation agent in a solvent, wherein the hydroxyl-protecting agent is borax, the hydroxyethylation agent is ethylene oxide, the solvent is selected from water, methanol and ethanol, the reaction temperature is 30-50°C and the reaction time is 4-14 hours; in the step of purification of 7-monohydroxyethyl rutoside, a recrystallization method is used for purification, wherein the solvent for recrystallization is selected from water, methanol, ethanol, isopropanol, and a mixture thereof, and wherein the recrystallization method includes dissolution, crystallization and filtration; in the step of preparation of trihydroxyethyl rutoside from purified 7-monohydroxyethyl rutoside, a hydroxyethylation agent is added after dissolving or dispersing 7-monohydroxyethyl rutoside in a solvent, wherein the hydroxyethylation agent is ethylene oxide, the solvent is selected from water, methanol, ethanol, pyridine and a mixture thereof, a catalyst is selected from sodium hydroxide, potassium hydroxide and ammonium water, the reaction temperature is 50-80°C and the reaction time is 3-8 hours, and the product is passed through cation and anion resins, or is treated with macroporous resin after the reaction is complete; wherein said cationic resins are selected from strong acidic polystyrene cation exchange resins, such as 007X7, 001X12, 001X1 and 001X16, the anion resins are selected from strong basic polystyrene anion exchange resins, such as 201X4 and 201X7; wherein said macroporous resins are selected from non-polar polystyrene macroporous resins, such as D101, Shanshi101, and low polar macroporous resins, such as D A, DAB-8 and D301; in the step of purification of trihydroxyethyl rutoside, a recrystallization method is used for purification, wherein a solvent for recrystallization is selected from water, methanol, ethanol, isopropanol, and a mixture thereof, and wherein the recrystallization method includes dissolution, crystallization and filtration.