The present invention relates to a method for preparing a positive electrode active material, including: a first step of adding to a reactor, a reaction solution including a transition metal-containing solution containing at least one among nickel, cobalt, and manganese, an ammonium ion-containing solution, and a basic aqueous solution to form seeds of precursor particles; a second step of preparing carbon-introduced precursor particles by adding a carbon source to the reactor when the precursor particles grow until an average particle diameter (D 50 ) of the precursor particles is 30% in size of the average particle diameter (D 50 ) of the finally prepared precursor particles; and a third step of mixing the carbon-introduced precursor particles and a lithium raw material and sintering the mixture at a temperature of 750 °C to 950 °C to prepare positive electrode active material particles, wherein the carbon introduced to the precursor particles is volatilized by the sintering of the third step to form cavities in the positive electrode active material particles, and a cavity ratio of the positive electrode active material is 5% to 20%.