The interface energies of different Ag(111)/MgF2(001) film interface structures are calculated based on the first-principle of density functional theory, and the stability of film interfaces is analyzed. The Ag(111)/MgF2(001) interface model with Ag diffusion is built to study the Ag diffusion behavior and mechanism at the interface. The influence of Ag diffusion on optical property of films is analyzed. The results show that the interface structure of the built smooth-interface-model is stable. The Ag diffusion at the Ag(111)/MgF2(001) interface mainly occurs at the surface/subsurface layers. The interaction between the diffused Ag and nearby F and Mg is weakly covalent. The Ag diffusion makes film optical absorption increase and the refractive index decrease, especially in the visible light wavelength region, the reduction of the refractive index is more obvious.