Using polymer brushes to guide the growth of gold nanostructures is of great scientific and technological importance. We report a new strategy to tune the shape of gold nanoparticles (NPs) on poly(2-vinylpyridine) (P2VP) brushes through vapor deposition polymerization of pyrrole: P2VP brushes loaded with AuCl₄⁻ ions were exposed to pyrrole vapor, one-step synthesized gold NPs and conducting polymer polypyrrole (PPy). The spiky gold NPs were obtained by controlling the thickness of P2VP brushes and reaction time, and a possible mechanism was proposed. The as-obtained spiky gold NPs could be used as substrates for surface-enhanced Raman scattering (SERS). They exhibited excellent Raman enhancement performance and the enhancement factor (EF) was in the range (4.0–6.2) × 10⁵ when 4-aminothiophenol was used as a probe molecule.