A method for purifying by-product ethylene glycol of polyester in converting plasticizer includes: inputting a reaction liquid waste stream including ethylene glycol and 2-ethylhexanol into an ethylene glycol dehydration tower; inputting a hot water stream into the ethylene glycol dehydration tower to mix with the reaction liquid waste stream and to remove the 2-ethylhexanol from the reaction liquid waste stream; dehydrating the ethylene glycol via the ethylene glycol dehydration tower to collect a crude ethylene glycol stream including dehydrated ethylene glycol from a tower bottom of the ethylene glycol dehydration tower and to collect an organic liquid waste stream including the 2-ethylhexanol from a tower top of the ethylene glycol dehydration tower; and inputting the crude ethylene glycol stream into an ethylene glycol distillation tower to collect a ethylene glycol solution from a tower top of the ethylene glycol distillation tower.