In this work, well-crystallized Au nanoparticles with avarage sizes of 30 ~35 nm were obtained through Galvanic replacement, and subsequently the Au@SiO2 composite particles with different silica shell thicknesses were successfully synthesized. The microstructures and morphologies of the Au@SiO2 nanoparticles were also characterized using UV-Vis, TEM and HRTEM. The Rhodamine 6G (R6G) dye was selected as probe molecules to evaluate the SERS effects of Au@SiO2 nanoparticles with various shell thicknesses. The results show that SRES signal intensity and quality of the R6G molecules are obviously improved as compared with Au NPs synthesized using the same method. Furthermore, the SiO2 shell thickness has a significant influence on the SERS acitivity. The highest SERS effect is abtained using Au@SiO2 nanoparticles with shell thickness of 2nm with an extremely low detection limit of 10-7 mol/L.