Abstract: In the paper,the preparation of ZnO/TiO2 multi layer film with S G method is introduced and the gas sensitive optical property of the film is studied.The film is very sensitive to ammonia.It's light transmittance increases considerably and monotonously as the gas concentration increases.The sensitive wavelength band extends to full visible spectrum.This material is of great practical value for gas sensing fiber.The gas sensitive mechanism and high selectivity mechanism are also discussed.