The invention relates to a method for online recycling PET polymerization waste. The method comprises the following main steps: (1) polymerization waste generated in a production process of PET is crushed, and the polymerization waste and tower bottoms of an esterification technology tower are added to a depolymerization kettle; (2) a vapor mixture in an esterification reaction kettle is supplied to the depolymerization kettle until the pressure in the kettle is balanced with the pressure of a polymerization esterification system; (3) a depolymerization reaction is performed under high temperature and high pressure conditions to obtain a low-molecular-weight PET depolymerization substance with the intrinsic viscosity of 0.05-0.15dL/g; (4) the depolymerization substance is loaded to an esterification system and subjected to pre-condensation, final polycondensation, pelleting and the like to prepare a polyester chip. The method realizes whole online recycling of the PET polymerization waste; the performance of obtained recycled polyester is equivalent to the performance of conventional polyester; the production cost of PET is significantly lowered.