The invention discloses a 3,4-difluorobenzonitrile preparation method. The 3,4-difluorobenzonitrile preparation method comprises the following steps of taking 3,4-difluorobenzonitrile as a raw material, potassium fluoride as a fluorinated reagent and bis-(N,N'-1,3-dimethyl-2-imidazolinyl)-chloride ammonium salt as a phase transfer catalyst, then adding a reducing agent and a dispersant, and performing reaction to prepare the 3,4-difluorobenzonitrile. In the method, the temperature of fluorination reaction is relatively low, the reaction time is short, the total molar yield can reach about 90 percent, and the solvents and the catalyst can be recycled; produced potassium chloride in reaction becomes a pure product with the purity being 99 percent or above after recrystallization, and the pure product can be directly sold, so that the production cost is reduced; in addition, the technology is safe and environmentally friendly, is small in quantities of three wastes (waste water, waste solid and waste gas) and is suitable for industrial production.