CH3NH3PbIxBr3-x thin films on FTO substrate were synthesized successfully. The crystal structure and optical properties of these films changed with different ratios of I-/Br- ions. The lattice parameters were reduced and the optical absorption edges were blue-shifted with the increasing ratio of Br- ions. The dielectric spectra have revealed that tetragonal CH3NH3PbIxBr3-x films at room temperature not only have a lower symmetry than cubic films, but also provide more electron transition energies. The mechanism is discussed correspondingly.