An electrolyte for a lithium secondary battery includes lithium salts, a non-aqueous organic solvent, and additive compounds, which initiates decomposition at 4V to 5V and show a constant current maintenance plateau region of more than or equal to 0.5V at measurement of LSV (linear sweep voltammetry). The additive compounds added to the electrolyte of the present invention decompose earlier than the organic solvent to form a conductive polymer layer on the surface of a positive electrode by increased electrochemical energy and heat at overcharge. The conductive polymer layer prevents decomposition of the organic solvent. Accordingly, the electrolyte inhibits gas generation caused by decomposition of the organic solvent during high temperature storage, and also improves safety of the battery during overcharge.