The invention discloses an environmentally-friendly preparation method of high-purity tetrahydrocarbazole. The method comprises the following steps: mixing cyclohexanone with methanol/ethanol or a mixed solution comprising methanol and ethanol, carrying out heating reflux, letting in a dry HCl gas as a catalyst, slowly adding phenylhydrazine and methanol/ethanol or the mixed solution comprising methanol and ethanol, and continuously refluxing; cooling the obtained reaction solution after reaction ending to slowly precipitate products from methanol/ethanol or the mixed solution comprising methanol and ethanol in microcrystal, adding distilled water for further precipitating products, filtering the products, washing the products with distilled water to neutrality, and drying to obtain high-purity tetrahydrocarbazole, wherein the mass ratio of the added distilled water to methanol/ethanol or the mixed solution comprising methanol and ethanol is 1:0.8-1:1.5, the mass ratio of phenylhydrazine to methanol/ethanol or the mixed solution comprising methanol and ethanol is 1:5-10, and the mass ratio of phenylhydrazine to cyclohexanone is 1:1.1-1.5. The preparation method has the advantages of simple process, economy, simplicity, mild reaction condition, short reaction time, high product yield, high product purity, environmental protection and the like.