A method for preparing a positive electrode active material is provided. The method includes a first step for 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 for preparing carbon-introduced precursor particles by adding a carbon source to the reactor when the precursor particles grow until the average particle diameter (D50) is 30% in size of the average particle diameter (D50) of the finally prepared precursor particles; and a third step for 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.