We report a facile method to synthesise flexible 3D surface-enhanced Raman scattering (SERS) substrates, using poly-m-phenylenediamine/polyacrylonitrile (PmPD/PAN) nanofiber mats as templates to self-assemble citrate-stabilized Ag nanocrystals (AgNCs), such as Ag nanoparticles (AgNPs), Ag nanotriangles (AgNTs) or Ag nanodisks (AgNDs). The SERS performances of AgNC@2D and AgNC@3D substrates were compared using 4-MBA as the probe molecule. The effect of the extinction wavelength as well as the density and morphology of the AgNCs on the SERS enhancement effect was explored. The results suggest that the 3D AgNT@PmPD/PAN nanofiber mat exhibits the highest SERS sensitivity and the lowest relative standard deviation (RSD) value. The detection limit of 4-MBA is as low as 10(-8) M, making the nanofiber mat a promising candidate for the SERS detection of chemical molecules. In addition, this study provides a simple route for the fabrication of SERS substrates with different types of noble metals and materials with strong SERS enhancement performance.