A positive electrode active material includes an active substance, where the active substance includes a lithium nickel manganese oxide, the lithium nickel manganese oxide includes a spinel phase, an X-ray diffraction pattern of the lithium nickel manganese oxide has a first diffraction peak at a diffraction angle 2θ of 18° to 19° and a second diffraction peak at a diffraction angle 2θ of 44° to 45°, the first diffraction peak corresponds to a crystal plane (111), and the second diffraction peak corresponds to a crystal plane (400); a peak intensity of the first diffraction peak is I(111), a peak intensity of the second diffraction peak is I(400), and I(111) and I(400) satisfy 2.5≤I(111)/I(400)≤4.0; and the lithium nickel manganese oxide satisfies a chemical formula LixMyNizMn2-y-zO4-k, where 0.8≤x≤1.05, 0≤y≤0.2, 0.3≤z≤0.7, −0.1≤k≤0.5, and M includes one or more of B, Si, P, S, Nb, Mo, Ru, Te, Ce, Ta, and W.