A surface-enhanced Raman scattering (SERS) substrate based on polymeric flat-plate waveguide and gold nanoparticles (AuNPs) is developed. The SERS signal of rhodamine 6G (R6G) is detected in the long range, and the enhancement performance and reproducibility of the substrate are analyzed. The experimental results show that the SERS signal increases and then decreases with increasing detection distance, reaching a maximum at a detection distance of 10 mm; the detection limit of the substrate for R6G is 10^-7 mol/L and the enhancement factor is 4.3×10⁴; the relative standard deviation (RSD) of each characteristic peak intensity of R6G is around 3% and has good repeatability. The polymeric flat-plate waveguide-AuNPs substrate can effectively detect SERS signals in a long range, laying the foundation for the non-destructive detection of molecules in biological samples.