The central wavelength of the polarization modes will separate obviously when the multiple cavities narrowband thin-film filter is in oblique incidence, which will cause the rapid increasing of the polarization dependent loss. It will seriously affect the optical communication system performance. The reason of the polarization dependent loss in oblique incidence is analyzed theoretically. By optimizing the thin-film stack, the central wavelength of the two polarization modes can be calibrated and the polarization dependent loss in the passband of the filter can be restrained effectively. Meanwhile, by using the polarization beam-splitters, the polarization dependent loss in the whole bandwidth of the filter can be also restrained. The experimental results show that these two methods can restrain the polarization dependent loss of the narrowband filter within 0.2 dB in oblique incidence, and they can be used in different occasion according to different practical demand.