The invention discloses a synthesis method of 3, 4, 5-trifluorophenol. The synthesis method includes following steps: taking 3, 4, 5-trifluorobromobenzene as a raw material, adding a certain volume ofammonia water and cuprous complexing catalyst, allowing heat-insulating reaction in a high-pressure kettle, and performing aftertreatment to obtain 3, 4, 5-trifluoroaniline; allowing 3, 4, 5-trifluoroaniline to be salified with sulfuric acid, being in diazotization reaction with nitroso-sulfuric acid, and hydrolyzing under copper salt catalysis to obtain 3, 4, 5-trifluorophenol. A hydrolysis device is introduced into a three-stage dichloromethane extraction device. Sulfuric acid concentration of a system is ensured to be constant, so that reaction yield is increased substantially. The synthesis method is easy-to-obtain in raw material and simple in process; the catalyst and the sulfuric acid are used for several times, so that raw material cost is low; the synthesis method is easy for industrialization and has good application prospect.