Embodiments of the present invention provide a lithium secondary battery electrolyte including a lithium salt, an organic solvent, and a flame retardant, and the flame retardant includes (pentafluoro) cyclotriphosphazene substituted by an electron donating group and (pentafluoro) cyclotriphosphazene substituted by an electron withdrawing group. Both two flame retardants: the (pentafluoro) cyclotriphosphazene substituted by the electron donating group and the (pentafluoro) cyclotriphosphazene substituted by the electron withdrawing group are added to the lithium secondary battery electrolyte, so that a battery has both high safety performance and good electrochemical performance. The embodiments of the present invention further provide a preparation method for the lithium secondary battery electrolyte and a lithium secondary battery including the lithium secondary battery electrolyte.