This research focused on studies of the effect of terephthalic acid (TPA) addition at the initial feed on diethylene glycol formation (DEG) in the process of polyethylene terephthalate (PET) preparation. Experimental results show that, when the mol ratio of ethylene glycol (EG) with TPA ranges from 1.0 to 1.3, the greater the amount of TPA added at the initial feed, the more DEG formed, and the greater the suppression of the rate of esterification and polymerization. In addition, the kinetics of EG with protons in the etherification reaction was considered in the study. It is found that, during the etherification reaction, the reactivity of hydroxyl end groups with ether bonds is higher than that of hydroxyl end groups on their own, and the activation energy of EG with protons is much lower than that of pure EG on its own.