To obtain a biodegradable polymer material with satisfactory thermal properties, higher elongation and modulus of elasticity, a new copolyester, poly(butylene succinate-co-ethylene succinate-co-ethylene terephthalate) (PBEST), was synthesized via direct polycondensation from three prepolymers of butylenes succinate, ethylene succinate and ethylene terephthalate (ET). The resulting copolyesters, PBEST, were characterized by 1 H-NMR, DSC, TG and WAXRD, and their melting temperature (T m ), melting heat of fusion (ΔH m ), glass-transition temperature (T g ), and thermal decomposition temperature (T d ) (1.5 wt%) were obtained. Compared to poly(butylene succinate-co-ethylene succinate) (PBES), PBEST has improved thermal properties such as higher T m and T d due to the incorporation of poly(ethylene terephthalate) unit into the main chains of copolyesters, but very low crystallization speeds. The degradation test of copolyesters in a compost condition shows that the degradability of PBEST is as a function of content of ET.