Abstract: The present application discloses a positive electrode active material, comprising a lithium nickel manganese oxide modified material and a coating layer on the surface of the lithium nickel manganese oxide modified material. The lithium nickel manganese oxide modified material is a primary particle with a core-shell-like structure comprising a spinel phase and a rocksalt-like structure phase. The spinel phase is an inner core, and the rocksalt-like structure phase constitutes an outer shell. The rocksalt-like structure phase is further doped with a phosphorus element and the phosphorus element is distributed in a gradient from the outer surface to the interior of the rocksalt-like structure phase. The present application further discloses a preparation method of the positive electrode active material, a positive electrode containing the positive electrode active material for lithium-ion secondary batteries, and a lithium ion secondary battery.