The distortion of cut-off gradient is a difficult problem in the practical preparation of band pass filter. Through admittance matching optimal design between the equivalent refractive index of periodic film structure and the substrate, the regular structure of band pass filter film is obtained. Based on the analysis of the variation of process parameters during the preparation of the film, it is proposed that the main reason for the gradient deformation is the error separation of the cut-off band caused by the change of the evaporation source characteristics of the electron gun. After correcting the error, a wide band-pass interference cut-off filter film with gradient change less than 25 nm is fabricated.