An optimized double resonance system consisting of a silver grating and silver nanoparticles is achieved through analyzing the extinction spectra and the field enhancement simulated by the FDTD method. The redshift of resonance positions of the Ag grating provides a chance for it to match the resonance of the Ag nanoparticles. Regarding Ag nanoparticles with a diameter of 50 nm and a gap distance of 2 nm, 520 nm is determined as the optimized period for the Ag grating to obtain the strongest field enhancement under excitation wavelengths of 532, 633, and 785 nm. The ridge width of the grating has little effect on field enhancement for the hybrid structure, so 258 nm is chosen as the ridge width for the grating with a period of 520 nm. A 4 nm thick gold layer is added on the surface of the Ag grating in simulation to improve its chemical stability and the better field enhancement is shown. The proposed Ag-Au grating/Ag nanoparticles system can be potentially used in SERS researches because of its high sensitivity and chemical stability.