The invention discloses a lipoic acid impurity, a preparation method of the lipoic acid impurity and the application of the lipoic acid impurity. The preparation method comprises the following steps that (1) lipoic acid is dissolved in a solvent, and H2O2 is added into the solution to react; (2) the product obtained in the step (2) reacts at the temperature of 20 to 25 DEG C, and the reaction process is detected by TLC (Thin-Layer Chromatography); (3) after the detection result of TLC that the raw material completely disappears is obtained, a proper amount of MnO2 is added into the reaction system; (4) after the H2O2 is completely decomposed, diatomite is adopted to filter, and the filtrate is subjected to decompression concentration; (5) the residue obtained in the step (4) is separated by a silica gel column to obtain yellow liquid, wherein PE (polyethylene)/EA is equal to 5:1; (6) the yellow liquid is subjected to rotary evaporation at room temperature to extract the solvent to obtain a straw yellow solid. The degradation mechanism and the degradation way of the lipoic acid oxidization impurity can be known, an impurity preparing method and a monitoring condition are provided, controllability of the impurity in a product production process and a product storage process is guaranteed, and the defect that the preparation method of the impurity and a monitoring method of the impurity in the market product are not available are filled.