A physical model was proposed to illustrate the influence of metal film thickness on electrical mean free path, and the formula of electrical mean free path was modified based on this model. The results demonstrate that the electrical mean free path decreases with film thickness decreasing, when the film thickness is less than the mean free path. Otherwise it is a constant as bulk material. The film conductivity expression was also modified based on this formula. Combining with the relationship between metal films′ conductivity and reflectivity, the formula of thin film conductivity with different thickness was put forward. Computer simulations show that when the film thickness is less than the electrical mean free path, the changes of metal film reflectivity are nonlinear.