We study theoretically surface band structure and topological phase transition for Bi2Se3 thin films under an external electric field. It is demonstrated that an electric field vertical to the film surface can close or reopen a gap for surface states. We also study the spin-dependent transport property through an interface junction between regions without and with electric field on the film. Interestingly, the opacity and transparency of spin-down Dirac fermions through the junction can be modulated by changing the incident angle and the electric field strength. On the contrary, the spin-up Dirac fermions always penetrate through the junction.