The invention relates to a preparation method of lithium tetrafluoroborate, which is characterized in that borane and fluorine gas react to generate boron trifluoride, and the boron trifluoride reacts with halogenated lithium salt dissolved in hydrogen fluoride to generate lithium tetrafluoroborate. The method comprises the following steps: putting LiF in a stainless steel reaction vessel, sufficiently dissolving the LiF in HF, and introducing a certain amount of borane, wherein the mole ratio of the boron element in the borane to the lithium element in the halogenated lithium salt is (1.2-1):1; slowly introducing fluorine gas, sufficiently reacting, and continuing stirring for 2-6 hours, wherein the reaction temperature is -20-15 DEG C; and after the reaction finishes, concentrating the reaction solution to 40-80% of the original volume by heating, filtering to obtain a LiBF4 crystal, and drying to obtain the lithium tetrafluoroborate product.