The present invention relates to a method of injection molding a polyethylene terephthalate (Polyethylene terephthalate, hereinafter 'PET' & quot;) the preform (Preform). An object of the present invention is reducing the amount of acetaldehyde in the injection-molded article which is generated by the injection molding of PET preforms by side reaction such as thermal decomposition and oxidation of the PET polymer due to the high molding temperature and addition, PET chips completing the solid-phase polymerization to shorten the time of the drying step is subjected before shaping process. In order to achieve the same purpose, in the present invention, after the solid phase polymerization rough PET chips dried by the addition of diethylene glycol to the PET chips, when injection molded, diethylene glycol acts as a plasticizer, since the molding temperature decreased acetaldehyde the amount of is reduced. In addition, the solid phase being added to diethylene glycol in PET chips after polymerization from the cooling unit, diethylene glycol is coated on the PET chips reduces the rate at which the combination of PET and moisture can be reduced the drying time of the PET chip subjected before shaping .