Aquatic ecosystem and human health have been seriously threatened by illegal discharge of wastewater, while simple and effective monitoring methods are still sparse. Here, we propose a facile method for on-site pollutant monitoring by surface enhanced Raman spectroscopy (SERS) with a novel substrate. This substrate is fabricated by interface self-assembly of Au@Ag nanocubes (NCs) on a simultaneously formed polyvinyl chloride (PVC) template, followed by coating a thin Au film. The Au@Ag@Au-NCs/PVC film is flexible, ultra-light and robust, which could float on water surface and firmly contact with water even under harsh environmental conditions. Moreover, the Au@Ag@Au-NCs/PVC film is translucent, allowing penetration of laser beam and enhancement of Raman signals. When thiram was used as model contaminant in the aqueous solution, a good linear relationship (R²= 0.972) was obtained over the range of 0.1-50 ppb with a detection limit of 0.1 ppb. Raman signals of thiram can be instantly and consecutively detected with the enhancement of the film in the simulated experiments, suggesting its possible use in the long run. Furthermore, the film can be easily regenerated by NaBH₄ solution washing, which could reduce the operating cost. In summary, the Au@Ag@Au-NCs/PVC film has great potentials in on-site pollutant monitoring in aqueous environment with a portable Raman spectrometer.