Abstract: Surface enhanced Raman spectroscopy(SERS) is an ultrasensitive vibrational spectroscopic technique to detect molecules. At present, adding salts is a main way to induce nanoparticles aggregation to get giant enhancement. However, this method needs more procedures and salts may etch nanoparticles. Here an effective and simple method is reported to enhance SERS effect by the aggregation of silver nanoprisms(AgNPRs) induced by 785 nm laser. Silver nanoprisms were prepared by ligand-assisted chemical reductions method. AgNO3 were reduced by NaBH4 in the presence of trisodium citrate, poly(vinylpyrrolidone) and H2O2.The edge of silver nanoprisms is about 80 nm. Surface plasmon band of silver nanoprisms is around 774 nm which could effectively absorb 785 nm laser. When laser irradiating silver nanoprisms during the detection of Raman spectra, these nanoprisms gradually aggregated and SERS spectra of analytes(4-mercaptobenzoic acid, 4-MBA) were gradually enhanced. And enhancement factor of Raman spectra ~109 is obtained by this method. Due to the huge magnitude of SERS in the near infrared region(excitation wavelength 785 nm), this technique has the potential in the field of biochemical tests.