The present disclosure relates to a method and an apparatus for decomposing a phenolic by-product generated in a bisphenol A preparation process, the method including: a step (S10) of feeding the phenolic by-product to a multistage reactive distillation column; a step (S20) of separating the phenolic by-product into an upper discharge stream containing an active component, a side discharge stream containing acetophenone, and a bottom discharge stream containing tar by the multistage reactive distillation column; and a step (S30) of mixing the side discharge stream discharged from the multistage reactive distillation column and the bottom discharge stream discharged from the multistage reactive distillation column to form a mixed discharge stream.