Abstract: To improve the thermal stability and mechanical performance of poly(vinyl acetate) (PVAc) emulsion adhesive, polyurethane (PU) derivatives were synthesized to prepare PVAc emulsion adhesives with core-shell structure by semi-batch seeded emulsion polymerization. PU derivatives were designed as soft shell and the PVAc as the hard core. Meanwhile, the effects of molecular weights of PU derivatives on the microstructure, thermal stability and mechanical performance of PVAc emulsion adhesive were investigated. The molecular weights of PU derivatives changed with the molecular weights of poly(ethylene glycol). The Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), atomic force microscope (AFM) and particle size analyzer were employed to determine the chemical composition, microstructure and the surface roughness of PVAc emulsion adhesives. Gel content and differential scanning calorimeter (DSC) were used to analyze the relationship between the structures and properties of PVAc adhesives. The thermogravimetric analysis (TGA), 180° peel force, loop tack and shear resistance were performed to identify the thermal stability and mechanical properties of PVAc emulsion adhesives. The results showed that with the increasing of the molecular weights of PU derivatives, the gel content and particle size of PVAc emulsion adhesive increased, and the thermal stability and adhesion strength of PVAc emulsion adhesive greatly enhanced, while the glass transition temperature (T_g) decreased. However, if the molecular weight of PU derivative was too big, the thermal stability and adhesion strength of PVAc emulsion adhesive decreased. Therefore, a PU derivative with optimal molecular weight to modify the PVAc emulsion adhesive was found, and the most proper molecular weight of poly(ethylene glycol) was 600.