PS@Au core-shell composite particles with excellent catalytic performance, high sphericity and no agglomeration were controllably synthesized by a suitable electroless plating process. The diameter of PS@Au composite particles could be adjusted continuously through tuning diameter of polystyrene microspheres. The influence of diameter on catalytic performance of PS@Au composite particles was investigated by SEM, XRD, UV-Vis spectrometer, and the detection of catalytic performance. The results indicate that the high specific surface area is responsible for the improvement of degradation rate of methylene blue chromophore and the excellent catalytic performance of PS@Au core-shell composite particles. Moreover, the diameter has a significant effect on both surface morphology and growth mode of Au nanoparticles, and the catalytic performance of the composite particles. When the diameter of core-shell structure is small, the curvature of the polystyrene microspheres is huge. Au nanoparticles exhibit isolated and granular structure because the orientation distribution of Au nanoparticles is diffused. In this case, PS@Au composite particles show the superior catalytic performance due to the higher specific surface area and surface energy. On the contrary, with the diameter of core-shell structure increasing, the curvature decreases. Au nanoparticles with close orientation merge and grow up rapidly, which is a film-like material. As a result, specific surface area and surface energy are decreased as the diameter rises and their catalytic performance also declines. In this study, the microstructure of Au nanoparticles begins to change from granular structure to continuous film-like structure, when the diameter of PS@Au core-shell composite particles is up to 400 nm