Different size-scale Ag nanoparticles were produced by Nd:YAG pulse laser with 532 nm wavelength and Excimer(KrF) pulse laser with 248 nm wavelength in redistilled deionized water,and two kinds of Ag colloids were obtained.The production of Ag nanoparticles is physical process,so the two kinds of Ag colloids are very pure.Through the detection of transmission electron micrograph(TEM),the Ag nanoparticles have homogeneous size distribution with good dispersibility.Without any extraneous components and residual oxidation products in the Ag colloids,we obtained much higher quality SERS spectra of p-hydroxybenzoic acid (PHBA) molecules in this Ag colloid,and the adsorption behavior was analysed in detail by simulations.