According to the characteristics of a thin film narrowband filter at oblique incidence, an angle-tunable narrowband filter with stable transmission peak and bandwidth can be designed by improving the structure of spacer film system to adjust its equivalent refractive index. According to the eight standards related to the dense wavelength division multiplexing (DWDM) system such as channel frequency, insertion loss, bandwidth, isolation degree, polarization dependent loss (PDL) and so on, in combination with the theory of thin film matrix and the newly constructed evaluation function, a tunable filter membrane system is constructed, and the optimal membrane systems satisfying different requirements are obtained by changing the weighting factor of each parameter. Using the built system, the angle-tunable narrowband filters with high isolation and low polarization-dependent loss for 100 GHz DWDM system on O-band and C-band are designed. The maximum tuning angle of the film system on C-band is 11°, and the isolation degree of adjacent channels is up to -30.3 dB. Finally, the transmission characteristics and the factors influencing the tuning angle limit are analyzed.