All-dielectric film narrowband filter is widely used in space-borne multispectral remote sensor owing to its excellent optical capability, manufacturability and environmental adaptability. But center wavelength shift, due to entrance light cone angle, affects its spectral selective power seriously. In order to study effects of incident light cone on transmission characteristics of narrowband filter, an informal dielectric film narrowband filter is designed and manufactured. Changes of transmission characteristics with oblique incidence of Gaussian beam of uneven illumination are analyzed. The count model of effective transmission with vertical incidence light cone is established and verified by spectral calibration. Results show that blue shift of central wavelength is the main influence of incidence light cone on transmission characteristics of all-dielectric film narrowband filter. Prediction accuracy of transmittance calculated by effective transmission model is better than 0.15% of central wavelength. Therefor, count model of effective transmission can be used to calculate the actual transmittance of film narrowband filter with oblique incidence of Gaussian beam, and spectral selective ability of narrowband filter will be improved when the reasonable film design data corrected by center wavelength shift is utilized during film coating. This provides a new technical approach to solve the problem of the wavelength shift of film narrowband filter with vertical incidence light cone.