By analysing the spreading of extraordinary ray in uniaxial crystal when the optical axis is parallel to the incident plane, the extraordinary ray reflection and transmission properties in uniaxial crystal film were studied.Based on the refractive index surface of uniaxial crystal and the relationship between extraordinary ray and wave normal and according to the phase matching condition of incident light, reflected light and refracted light, the boundary conditions at the surfaces of uniaxial crystal were obtained when optical axial is parallel with the incident plane.Then, the reflectance and transmittance of crystal thin film were calculated based on the phase relationship and the boundary conditions at the surfaces of two crystal thin films.The transmittance and the relationship between the change of refractive index with optical axis angle of quartz(positive crystal) thin film and lithium niobate(negative crystal) thin film were calculated by using the method proposed.The results show that the method is correct and suitable for both positive crystal and negative crystal.The formulation given in the paper is general and representative.