By synthesizing Au@SiO₂ nanoparticles with shell thickness of 2~3 nm and core size of 15, 25, 50 nm instead of Au nanoparticles, the relationship between electromagnetic coupling intensity and particle size and gap is studied. The results show that the Au@SiO₂ nanoparticles with core size 50 nm have much more enhancement. In order to further increase the surface-enhanced Raman scattering (SERS) activity, the Au@SiO₂ nanoparticles are layered on the smooth gold surface, and then stronger SERS signal is obtained than that from Au@SiO₂ nanoparticles on the silicon surface. The dimensional finite different time domain method is used to simulate the SERS activity under different size, gap and substrate conditions. The results show that the bigger of the particles are and the narrower the gaps are, the higher the electromagnetic coupling intensity of the Au nanoparticles on the smooth gold surface is. It agrees well with our experiment data. In addition, the hot spot from the gap between nanoparticles is transferred to the gap between nanoparticle and substrate. This provides a convenient way to prepare the high sensitivity and stability Raman substrate.