The core-shell acrylic emulsion was synthesized，with P(MMA/BA) as core，and with P(2-EHA/MBA/HPA) as shell. The influence of the mass ratio of core/shell on mechanical properties was investigated in detail. The high elongation at break proved that the emulsion may have a potential application in coating industry, afforded by HPA as functional monomer. However, when the amount of HPA exceeds 1.5 wt. % to the total monomers, the emulsion will become unstable and precipitation occurs.