A method for preparing a bimodal-type positive electrode active material precursor, the method comprising: step 1 of preparing a first transition metal aqueous solution and a second transition metal aqueous solution; step 2 of adding a first reaction raw material including the first transition metal aqueous solution, an ammonium cation complex forming agent, and a basic aqueous solution to a reactor, and then subjecting the mixture to a co-precipitation reaction under a primary pH condition to form nuclei of first positive electrode active material precursor particles; step 3 of adjusting the input amount of the first reaction raw material in order to adjust the pH in the reactor to be a secondary pH condition which is in a lower range than the primary pH condition, and growing the first positive electrode active material precursor particles; step 4 of adding a second reaction raw material including the second transition metal aqueous solution, an ammonium cation complex forming agent, and a basic aqueous solution to the reactor containing the first positive electrode active material precursor particles to allow the pH in the reactor to meet the primary pH condition, and then subjecting the mixture to a co-precipitation reaction to form nuclei of second positive electrode active material precursor particles; and step 5 of adjusting the input amount of the second reaction raw material in order to adjust the pH in the reactor to meet the secondary pH condition, and then simultaneously growing the first positive electrode active material precursor particles and the second positive electrode active material precursor particles to prepare a bimodal-type positive electrode active material precursor including the first positive electrode active material precursor particles and the second positive electrode active material precursor particles having different average particle diameters (D 50). step 1 of preparing a first transition metal aqueous solution and a second transition metal aqueous solution; step 2 of adding a first reaction raw material including the first transition metal aqueous solution, an ammonium cation complex forming agent, and a basic aqueous solution to a reactor, and then subjecting the mixture to a co-precipitation reaction under a primary pH condition to form nuclei of first positive electrode active material precursor particles; step 3 of adjusting the input amount of the first reaction raw material in order to adjust the pH in the reactor to be a secondary pH condition which is in a lower range than the primary pH condition, and growing the first positive electrode active material precursor particles; step 4 of adding a second reaction raw material including the second transition metal aqueous solution, an ammonium cation complex forming agent, and a basic aqueous solution to the reactor containing the first positive electrode active material precursor particles to allow the pH in the reactor to meet the primary pH condition, and then subjecting the mixture to a co-precipitation reaction to form nuclei of second positive electrode active material precursor particles; and step 5 of adjusting the input amount of the second reaction raw material in order to adjust the pH in the reactor to meet the secondary pH condition, and then simultaneously growing the first positive electrode active material precursor particles and the second positive electrode active material precursor particles to prepare a bimodal-type positive electrode active material precursor including the first positive electrode active material precursor particles and the second positive electrode active material precursor particles having different average particle diameters (D 50).