A novel surface-enhanced Raman scattering(SERS) method was established for the quantitative determination of trace phosmet via potassium thiocyanate(KSCN) serving as an internal standard.The silver colloid with high SERS activity，a kind of SERS substrate，was prepared by sodium citrate reduction.The enhanced Raman signal of phosmet on silver colloid was proved through comparing the normal Raman spectra of silver colloid and phosmet with SERS spectrum of trace phosmet.The concentrations of phosmet could be determined with the corresponding SERS spectra produced by adsorption of phosmet on nano silver colloid surface，while KSCN was selected as the internal standard.The strongest SERS signal of phosmet appeared at 503 cm-1 ,and was assigned to the S—P stretching vibration，so it was considered as quantitative peak.The range linear of 5.0×10-7-1.2×10-5 mol?L-1 was obtained for concentrations of phosmet divided by ratio of S—P stretching vibration at 503 cm-1 and SCN- characteristic peak at 2 120 cm-1.The linear correlation coefficient(r) was 0.997 8,and the detection limit(S/N=3) was 2.82×10-7 mol?L-1.Furthermore，the proposed method was applied in the detection of simulated water sample，with recoveries of 88.7%-110.2% and relative standard deviations(RSD) not more than 8.0%.The interference experiments were also considered by the detection of some inorganic ions and the background SERS signal of simulated water sample.With the advantages of simplicity，easy operation and no interference of internal standard substance，the proposed method could be extended to the determination of other pesticide molecules.