The invention provides a lithium ion secondary battery and an electrolyte thereof. The electrolyte of the lithium ion secondary battery comprises lithium salt, a non-aqueous solvent, and an addition agent at least containing 1, 3-propane sultone (PS), fluoroethylene carbonate (FEC) and an s-triazine structural compound, wherein the s-triazine structural compound is represented by a general formula (1), a general formula (2) or a general formula (3); in the general formula (1), the general formula (2) or the general formula (3), n is an integral number of 0-5, and R1, R2 and R3 are hydrogen atoms or C1-C6 alkyl group; and the hydrogen atoms on the alkyl group can be partially or wholly replaced by fluorine atoms. The lithium ion secondary battery comprises a positive plate, a negative plate, a diaphragm arranged between the adjacent positive plate and the negative plate, and the electrolyte of the lithium ion secondary battery. According to the lithium ion secondary battery and the electrolyte thereof provided by the invention, oxidation reaction between the electrolyte and a cathode material can be inhibited, and the cycle performance under the conditions of high temperature and high voltage is improved.