The invention relates to a method for preparing high-purity levocarnitine, belonging to a key technology for controlling the quality of different links in the process of producing chiral drug. The method comprises the steps of: testing the content of the laevo isomer impurity in S-epoxy chloropropane of chiral raw materials with a gas chromatograph and a chiral column; controlling the laevo isomer impurity in reaction raw materials to be a certain limitation; testing and controlling the specific rotation of chiral midbody L-3-chlorine-2-hydroxypropyl trimethylamine to be under a certain limitation with an optical rotation apparatus; testing the content of the dextroisomer in the midbody L-3-chlorine-2-hydroxypropyl trimethylamine jointly with (+) alpha-methyl-6-methoxyl-2-naphthalene acetylchloride derivatization reagent through a HPLC method; controlling the content of the optical isomer of the reaction midbody to be under a certain limitation; and the final product levocarnitine is tested by jointly with the (+) alpha-methyl-6-methoxyl-2-naphthalene acetylchloride derivatization reagent through the HPLC method. The testing and controlling method can obtain the high-purity levocarnitine, wherein the content of the laevo isomer can reach more than 97%, and the content of the dextroisomer can reach less than 2%.