In this paper, Au nanosphere and Au-Ag alloy nanourchin were prepared by reducing the chloroauric acid. The mixed colloid solutions of Au nanosphere and Au-Ag alloy nanourchin were used as surface-enhanced Raman scattering (SERS) substrate to detect polycyclic aromatic hydrocarbons (PAHs) in aqueous solution. The size of Au-Ag alloy nanourchin particle was about 300~400 nm and the thorn-like bulge covered on it was about 40~100 nm. The mixed colloid solutions of Au nanosphere and Au-Ag alloy nanourchin which were optimized pH values and other parameters presented a better enhancement than Au nanosphere. The enhancement effect was about three times that of Au nanosphere colloid solution. Three kinds of high molecular weight PAHs, pyrene(4 rings), benzoanthracene(4 rings) and benzo［a］pyrene(5 rings), were detected. The results showed that there were good linear relationships between Raman intensity and concentration in the low concentration range and the mixed SERS substrate had a good reproducibility and stability. Their limits of detection (LODs) were 0.44, 2.92 and 1.64 nmol·L-1, respectively. The innovation of this paper was that the mixed colloid solutions of Au nanosphere and Au-Ag alloy nanourchin are prepared as SERS substrate and the trace-level high molecular weight PAHs are detected. The results show that the detection of trace-level high molecular weight PAHs in aqueous can be realized using the mixed SERS substrate prepared in this study, which proposed an in-situ method for detecting the high molecular weight PAHs in aqueous.