The present disclosure provides a method for decomposing a phenolic by-product, the method including: a step S10 of injecting and mixing a bisphenol A by-product produced in a bisphenol A production process, a mixed by-product stream of phenol by-products produced in a phenol production process, a decomposition apparatus side discharge stream, and a process water stream in a mixing apparatus; a step S20 of injecting a mixing apparatus discharge stream discharged from the mixing apparatus into a phase separation apparatus and phase-separating the mixing apparatus discharge stream into an oil-phase stream and a liquid-phase stream; a step S30 of feeding the oil-phase stream, which is phase-separated in the step S20 and discharged from the phase separation apparatus, to a decomposition apparatus to decompose the oil-phase stream; and a step S40 of circulating the decomposition apparatus side discharge stream obtained by the decomposition in the step S30 to the mixing apparatus in the step S10.