The rational design of the sample cell may improve the sensitivity of surface-enhanced Raman scattering (SERS) detection in a high degree. Finite difference time domain (FDTD) simulations of the configuration of Ag film-Ag particles illuminated by plane wave and evanescent wave are performed to provide physical insight for design of the sample cell. Numerical solutions indicate that the sample cell can provide more "hot spots'' and the massive field intensity enhancement occurs in these "hot spots''. More information on the nanometer character of the sample can be got because of gradient-field Raman (GFR) of evanescent wave.