To realise the on-site, rapid and high sensitivity detection of trace polycyclic aromatic hydrocarbons (PAHs) in coastal water, a portable high-sensitive surface-enhanced Raman scattering sensor was developed, and a high-efficiency outside optical path of spectrometer assembled with inverted telescope structure was applied, which optimized the collection efficiency around 3.7 times better than a commercial optical probe, and the optical path was about 120 mm long. A high sensitive surface-enhanced Raman scattering (SERS) sensor was also established with 350 mm×300 mm×180 mm, 15 kg quality and 30 W power consumption, which can satisfy the portable requirements in on-site detection. PAHs in coastal sea water were detected on-site using this sensor, and signals of naphthalene, acenaphthylene, phenanthrene, chrysene, benzo (k) fluoranthene, benzo (b) fluoranthene, benzo (a) pyrene and so on were recognized. Meanwhile, the results were confirmed by the gas chromatography (GC). Besides, volatile naphthalene was discriminated by SERS but wasn′t recognized by GC. SERS detection of Shilaoren and Maidao area of Qingdao coastal area showed that in Shilaoren coastal area, with the increase of offshore distance, the contents and concentration of PAHs were similar, but in Maidao coastal area, the contents and concentration of PAHs reduced significantly. The results indicate that SERS sensor is portable, convenient and with high sensitivity, and can provide data for PAHs monitoring in the high pollution area.