Silver nanoparticles are synthesized through thermal evaporation for molecular detection using surface enhanced Raman scattering microscopy. The optical properties of silver nanoparticles are obtained by ultraviolet-visible spectrometry, which show the resonance wavelength near the detecting wavelength of Raman scattering (488 nm). Using rhodamine 6G as a test molecule, the results in this paper show that the detected Raman peak intensity has a nonlinear relationship with the incident power density when surface plasmon of silver nanoparticles was excitated by incident photon. This nonlinear phenomenon of surface enhanced Raman scattering caused by "hot spot" with high electromagnetic field strength provides an effective way to obtain high scattering intensity without high incident power density, which may expand the scope of Raman scattering application.