A polymer electrolyte for a lithium secondary battery that may include: a fluorine-based solvent; a lithium salt; and a flame retardant polymer containing phosphorus and fluorine, wherein the polymer electrolyte has a fluorine content of 35 to 60% by weight and a phosphorus content of 2.3 to 7.5% by weight. Lithium secondary batteries containing the polymer electrolyte exhibit improved flame retardancy and ionic conductivity as well as enhanced high-voltage stability.