Combined Ag nanoparticles with Al2O3 dielectric layer structure have been designed for molecular detection using surface enhanced Raman scattering. Optical absorption spectra studies reveal dipole plasmon resonance absorption property, which show regular red shift with the increasing of the thickness of Al2O3 layer. By use the combined structure as SERS substrates and rhodamine 6G as a test molecule, the results in this paper show that the scattering at 1064 nm increases with the thickness of Al2O3 dielectric layer which leads to the surrounding dielectric constant of Ag nanoparticles to increase.