The invention provides a method for preparing a positive electrode active material. The method is used for solving the problem that the positive electrode energy density of a lithium ion battery is low in the prior art. The method is characterized by including the following steps that S1, a precursor material with the molecular formula of (NH4)3[AlMo6O24H6] is obtained, and the average grain diameter of the precursor material is 19 microns or above; S2, the precursor material is dissolved in water, a water-soluble alcohol organic solvent is added under the stirring condition, recrystallization is carried out, filtering is carried out, and then the positive electrode active material is obtained; the volume ratio of the water-soluble alcohol organic solvent to water is (1-5):1. The invention further discloses a positive electrode piece and a lithium ion battery which contain the positive electrode active material prepared with the method. The positive electrode active material prepared with the method is small in average grain diameter and high in effective capacity per gram, and the energy density of the lithium ion battery can be improved.