The invention relates to a preparation method of 3-fluorophenol. According to the preparation method, m-aminophenol is used as a raw material, fluoro-substitution reaction is performed in an organic solvent by quantitatively using anhydrous hydrofluoric acid and KNO2 is used as a diazotization reagent, so that the environmental pressure brought by recovering hydrofluoric acid is alleviated; the organic solvent is recovered and reused, so that the environmental treatment pressure is alleviated to the maximum extent. The preparation method provided by the invention is low in cost of raw materials, mild in reaction condition and simple in equipment and provides a feasible process line for industrial production of 3-fluorophenol.