The invention belongs to the field of materials for lithium-ion batteries, and discloses a nickel-containing lithium-rich and manganese-based positive electrode material and a preparation method thereof. The method specifically comprises the following steps: preparing a solution from a nickel source, a manganese source, a doping element source, a complexing agent, a strong oxidant and a precipitator with a solvent; mixing the nickel source, the manganese source and the doping element source and adding the complexing agent under a stirring condition to obtain a complexing agent sediment; adding a strong oxidant solution and a precipitator solution to the complexing agent sediment under the stirring condition, and controlling the pH value of a system with aqueous ammonia until complete precipitation to obtain an intermediate sediment; and filtering, washing and drying the intermediate sediment and then mixing the sediment with the lithium source, and burning the mixture to obtain the nickel-containing lithium-rich and manganese-based positive electrode material. The invention further discloses a positive electrode and a battery prepared from the positive electrode material. According to the method disclosed by the invention, nickel elements in positive electrode material particles and on the surfaces thereof are fully oxidized, so that the uniformity and the stability of the positive electrode material are improved; and the cycling stability and the rate capability of the battery are improved.