The invention discloses a novel substance, namely (3aS,4S,6aR)-4-(5'-oxohexyl)- tetrahydrothiophene-[3,6-b]imidazoline-2-ketone and a synthesis method thereof. The method for preparing the novel substance comprises the following steps: under the protection of nitrogen, acetylacetic ether is dripped into sodium hydride, sodium alkoxide or metal sodium and an anhydrous solvent, the temperature is increased to be between 100 and 110 DEG C, and a reaction system is clarified and cooled to normal temperature; then sulfonium salt is added into the solution for a thermal insulation reaction at a temperature of between 70 and 90 DEG C, and then the solution is cooled to normal temperature; an organic layer is separated out, a yellowish liquid is obtained through vacuum recovery, then a hydrogen bromide solution is directly added into the yellowish liquid to be heated to reflux, and then the solution is cooled to normal temperature; and a white solid, namely a target product is obtained through the post-treatment. The novel substance provides the comparison of impurities in a pure product for the measurement of impurity content in biotin, thereby improving the quality of the biotin.