MECDP copolyesters based on poly(ethylene terephthalate) were prepared with sodium-5-sulfo-bis-(hydroxyethyl)-isophthalate and poly(ethylene glycol) units as modifiers. Thermal properties of these copolyesters were characterized by differential scanning calorimetry and thermal gravity analyzer. Experimental results indicated that glass transition temperature, melting temperature and thermal degradation temperature reduced with increasing the poly(ethylene glycol) content. The incorporation of poly(ethylene glycol) increased the flexibility and irregularity of molecular chains which led to lower crystallinity, and bought more ether bonds into molecular chains. Besides, the thermal degradation under oxygen condition happened easily compared to that under nitrogen condition.