In the present paper, the gold colloid with parameters optimized was used as surface-enhanced Raman scattering(SERS) active substrate to realize the trace detection of polycyclic aromatic hydrocarbons(PAHs) in water for the first time. Gold colloids with different size were prepared using chemical reduction method, and the optimum size selected at 632.8 nm excitation wavelength by experiment is (32±3) nm. The influence of pH value on the enhancement of PAHs was researched, and the optimal pH value is 13. Spectral intensity increased by approximately 20-fold compared with pH 6. The SERS spectra of naphthalene, phenanthrene and pyrene aqueous solutions were detected by the optimum gold colloid, and the minimum concentrations obtained were 20, 4 and 4nmol·L-1, respectively. There was a linear relationship between peak intensity and concentration, and the linear regression correlation coefficients were all above 0.985. For the mixture, the authors could distinguish each PAH easily for their own characteristic peaks. The experimental results show that such active substrate has a very high sensitivity as well as good application prospect.