By using laser excitation of three different photon energies, we have experimen tally studied the SERS spectra of pyridine molecules adsorbed on the surface of an electroohemically roughened silver electrode. The experiment shows the relation between SEES spectra intensity and the photon energy as well as the applied voltage. We introduce the viewpoint that the applied voltage has influence on the energy levels of adsorbed pyridine molecules and make improvement of the exising charge-transfer excitation theory of SERS spectra. The experimental results are explained in this way.