A method for preparing a positive electrode active material, comprising: a first step of adding to a reactor, a reaction solution comprising 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% or greater in size of the average particle diameter (D 50) of 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 in the third step to form cavities in the positive electrode active material particles, and a cavity ratio of the positive electrode active material is 5-20%, wherein cavities are empty spaces formed while carbon introduced to precursor particles is volatilized, the cavity ratio is a ratio of area for which the cavities account with respect to the cross-sectional area of the positive electrode active material particle and is measured as described in the specification, the carbon source is added so that the carbon compound is 20 vol% or less with respect to 100 vol% of total precursor particles, and the carbon compound has an average particle diameter (D 50) of 10 nm to 1 μm. a first step of adding to a reactor, a reaction solution comprising 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% or greater in size of the average particle diameter (D 50) of 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 in the third step to form cavities in the positive electrode active material particles, and a cavity ratio of the positive electrode active material is 5-20%, wherein cavities are empty spaces formed while carbon introduced to precursor particles is volatilized, the cavity ratio is a ratio of area for which the cavities account with respect to the cross-sectional area of the positive electrode active material particle and is measured as described in the specification, the carbon source is added so that the carbon compound is 20 vol% or less with respect to 100 vol% of total precursor particles, and the carbon compound has an average particle diameter (D 50) of 10 nm to 1 μm.