The invention discloses a method for preparing kynurenine from horseshoe crab tails.The method comprises the following steps of preparation of dry horseshoe crab tail powder, preparation of horseshoe crab tail crude extract, primary separation and purification of the horseshoe crab tail crude extract, separation and purification of a component A8 and separation and purification of components A8-24, wherein separation and purification of the components A8-24 comprise the steps that an appropriate amount of components A8-24 are taken, methyl alcohol is added for dissolution, and normal phase column separation is conducted; 50 mg of samples are taken and stirred with 150 mg of silica gel powder, 5 g of silica gel is saturated with petroleum ether and then subjected to column packing, after the the samples are packed in a column, elution is conducted with petroleum ether and acetone, when the ratio of petroleum ether to acetone is 1:10, fractions are collected, the number of the fractions is 60, the fractions from the 25 tube to the 40 tube are merged, after the the fractions are merged, vacuum concentration is conducted, and after solvent is dried out, kynurenine is obtained.According to the method for preparing kynurenine from the horseshoe crab tails, kynurenine is separated from the horseshoe crab tails for the first time, and kynurenine serving as the active ingredient of the horseshoe crab tails has great significance in secondary development of horseshoe crab tails.