The invention discloses a lithium-ion secondary battery and an electrolyte thereof. The electrolyte comprises a solvent, lithium salt and a film forming additive, wherein the solvent comprises a first solvent and a second solvent; the first solvent comprises linear carboxylic ester and ethylene carbonate; the second solvent is one or more of ethyl methyl carbonate, diethyl carbonate, dimethyl carbonate and propylene carbonate; and the film forming additive is one or more of fluoroethylene carbonate, vinylene carbonate, 1,3-propane suhone, succinonitrile, adiponitrile, lithium bis(oxalato)borate, and lithium oxalyldifluoroborate. Due to the collocation of linear carboxylic ester and ethylene carbonate, a solvent system with a higher dielectric constant and low viscosity is obtained; the film forming additive improves poor compatibility between linear carboxylic ester and graphite; and finally the lithium-ion secondary battery adopting the electrolyte presents high-power discharge capacity, excellent high-temperature cycling stability and low-temperature charge and discharge properties.