Disclosed are a flexible paper-based surface-enhanced Raman scattering (SERS) substrate and a method of preparing the same, which relate to detection of trace organics. This invention deposits a film of silver nanoparticles on a carrier such as filter paper by oblique-angle deposition to prepare the SERS substrate, which can be used for the detection of trace organics. Silver nanoparticles are closely arranged on the flexible paper, allowing the substrate for good SERS effect due to the interparticle coupling effect. Moreover, compared to the traditional rigid and brittle substrates, the substrate prepared herein can be readily used in the detection of residual organics on complex surfaces in various liquid or solid conditions by dipping, dropwise adding or wiping due to the use of a flexible and absorbent carrier.