A method for decomposing a phenolic by-product generated in a phenol preparation process, the method comprising: a step of adding a phenolic by-product stream, a decomposition apparatus side discharge stream, and process water to a mixing apparatus and mixing the phenolic by-product stream, the decomposition apparatus side discharge stream, and the process water; a step of adding a mixing apparatus discharge stream discharged from the mixing apparatus to a phase separation apparatus and phase-separating the mixing apparatus discharge stream into an oil phase and an aqueous phase; a step of feeding an oil phase stream obtained by the phase-separation in the step and discharged to a decomposition apparatus and decomposing the oil phase stream; a step of circulating the decomposition apparatus side discharge stream obtained by the decomposition in the step to the mixing apparatus in the step, the method further comprising: prior to the step, a step of performing an acid decomposition reaction on cumene hydroperoxide in the presence of an acid catalyst; a step of adding a basic aqueous solution to an acid decomposition reaction solution discharged in the step, neutralizing the acid decomposition reaction solution, and phase-separating the neutralized acid decomposition reaction solution into an oil phase and an aqueous phase; a step of adding an oil phase acid decomposition reaction solution separated in the step to a separation apparatus and separating the oil phase acid decomposition reaction solution into a separation apparatus upper discharge stream containing acetone and a separation apparatus bottom discharge stream containing phenol; and a step of adding the separation apparatus bottom discharge stream separated in the step to a phenol column and separating the separation apparatus bottom discharge stream into a phenol column upper discharge stream containing phenol and a phenol column bottom discharge stream containing a phenolic by-product; wherein the phenolic by-product stream is the phenol column bottom discharge stream; and wherein the decomposition apparatus side discharge stream contains 50 wt% or more of acetophenone. a step of adding a phenolic by-product stream, a decomposition apparatus side discharge stream, and process water to a mixing apparatus and mixing the phenolic by-product stream, the decomposition apparatus side discharge stream, and the process water; a step of adding a mixing apparatus discharge stream discharged from the mixing apparatus to a phase separation apparatus and phase-separating the mixing apparatus discharge stream into an oil phase and an aqueous phase; a step of feeding an oil phase stream obtained by the phase-separation in the step and discharged to a decomposition apparatus and decomposing the oil phase stream; a step of circulating the decomposition apparatus side discharge stream obtained by the decomposition in the step to the mixing apparatus in the step, the method further comprising: prior to the step, a step of performing an acid decomposition reaction on cumene hydroperoxide in the presence of an acid catalyst; a step of adding a basic aqueous solution to an acid decomposition reaction solution discharged in the step, neutralizing the acid decomposition reaction solution, and phase-separating the neutralized acid decomposition reaction solution into an oil phase and an aqueous phase; a step of adding an oil phase acid decomposition reaction solution separated in the step to a separation apparatus and separating the oil phase acid decomposition reaction solution into a separation apparatus upper discharge stream containing acetone and a separation apparatus bottom discharge stream containing phenol; and a step of adding the separation apparatus bottom discharge stream separated in the step to a phenol column and separating the separation apparatus bottom discharge stream into a phenol column upper discharge stream containing phenol and a phenol column bottom discharge stream containing a phenolic by-product; wherein the phenolic by-product stream is the phenol column bottom discharge stream; and wherein the decomposition apparatus side discharge stream contains 50 wt% or more of acetophenone. prior to the step, a step of performing an acid decomposition reaction on cumene hydroperoxide in the presence of an acid catalyst; a step of adding a basic aqueous solution to an acid decomposition reaction solution discharged in the step, neutralizing the acid decomposition reaction solution, and phase-separating the neutralized acid decomposition reaction solution into an oil phase and an aqueous phase; a step of adding an oil phase acid decomposition reaction solution separated in the step to a separation apparatus and separating the oil phase acid decomposition reaction solution into a separation apparatus upper discharge stream containing acetone and a separation apparatus bottom discharge stream containing phenol; and a step of adding the separation apparatus bottom discharge stream separated in the step to a phenol column and separating the separation apparatus bottom discharge stream into a phenol column upper discharge stream containing phenol and a phenol column bottom discharge stream containing a phenolic by-product; wherein the phenolic by-product stream is the phenol column bottom discharge stream; and wherein the decomposition apparatus side discharge stream contains 50 wt% or more of acetophenone.