Dependences of the equivalent Lorentz vibration and interband transition on the thickness and steady-state time relevant to the free and bound electrons in Ag thin films are discussed. Based on spectral transmittances of Ag films with different thicknesses and different steady states in the visible region, in combination with the effective medium theories of island films of metals. Theoretical calculations of the spectral transmittance agree with the experiments very well. By comparing the theories with the experiments, the optical constants of Ag films with, different tiucknesses and different steady states are obtained.