Spherical silver nanoparticles are prepared by chemical reduction of sodium citrate,and photoinduced conversion of spherical Ag nanoparticles is conducted by the irradiation of high pressure spherical Xe lamp. The spectra property and morphologies of silver nanoparticles prepared under different irradiation time are studied by UV-visible absorption spectroscopy and Transmission Electron Microscope(TEM). Using crystal violet as a molecular probe, SERS spectra of crystal violet on Ag nanoparticles are studied. Experimental results indicate that the color of Ag colloid changes observably and the number of absorption peak of Ag colloid gradually manifold with the increase of irradiation time. TEM pictures show that morphology of Ag nanoparticles is evolved from sphericity to triangular plate, finally truncated triangular plate. As the irradiation time increasing, the SERS signal intensity increases first, and then decreases.