In this paper, the SERS effect of pyridine on silver films with different thickness prepared by chemical deposition is studied. The experimental results show that geometry of metal surface plays an important role in SERS. The enhancement depends on not only the size and the separation of particles of silver surface, but also the ratio of the two. Finally, the waveguide resonance theory is used to explain the experimental phenomena. Our experimental results are in agreement with the zeroth order approximation calculation.