Provided is a method of decomposing phenolic by-products, and more particularly, a method of decomposing phenolic by-products including: supplying a phenolic by-product stream to a decomposition device to perform thermal decomposition; separating an upper discharge stream including effective components and a lower discharge stream including materials having a high boiling point in the decomposition device; supplying the lower discharge stream from the decomposition device, a side discharge stream from the decomposition device, and a process water stream to a mixing device and mixing these streams; and supplying a discharge stream from the mixing device to a layer separation device to separate the discharge stream from the mixing device into an oil phase and an aqueous phase.