The invention discloses a lithium bis(oxalate)borate preparation method, which comprises: adding a boron trifluoride dimethyl carbonate complex, lithium fluoride and an organic solvent into a reactionflask, mixing, heating while stirring, and carrying out a reaction; cooling the reaction solution to a room temperature, adding anhydrous oxalic acid into the reaction solution, heating under a stirring condition, adding silicon tetrachloride in a dropwise manner, continuously heating after the adding, and carrying out a reaction; filtering the reaction solution after the reaction, then concentrating the filtrate under a reduced pressure condition, adding an inert solvent into the filtrate, and carrying out stirring crystallization to obtain a lithium bis(oxalate)borate semi-finished product;and re-crystallizing and drying to obtain high-purity lithium bis(oxalate)borate. According to the invention, the method has the advantages of thorough reaction, reduction of the separation difficulty of the product and the by-product in the reaction process, improvement of the purity and the yield of the product, further reduction of the production cost, and improvement of the product quality.