Abstract: Polyethylene glycol-citric acid copolymer(PEGCA) was synthesized by polycondensation. Its blend with polylactide(PLA) was prepared via internal mixing. The morphology, thermal properties and mechanical properties of the blend were studied. It was found that PEGCA and PLA were partially compatible, and PEGCA was dispersed as particles in PLA matrix. PEGCA showed good toughening effect on PLA. When the mass fraction of PEGCA was 15%, the elongation at break and impact strength of the materials increased to 327.9% and 63.0 J/m, respectively. Meanwhile, the cavitation theory was used to explain the toughening mechanism, and the toughening effect was also described quantitatively by critical ligament thickness theory and integral J.