The central wavelength of the multi-c avity narrowband thin film filter will shift to short wavelength in oblique inci dence,and it can be fabricated as an angle tunable filter.The transmitted-facul ar of the narrowband filter will broaden obviously while the incident angle is i ncreassing.It will cause great insertion loss of the transmission spectrum and l imit its wavelength tunable range because the coupling degree and the interferen ce order will be decreased.Traditional beam spatial superposition method can sim ulate the phenomenon of the transmitted-facular broadening in oblique incidence .However,the simulation results have high errors relative to the experimental re sults.Based on the fast Fourier transform (FFT) and the Fresnel formula,a novel frequency domain recursive algorithm for calculating the transmitted-facular br oadening and the transmitted-intensity distribution is proposed in this paper.I t simuates the transmitted-facular broadening and transmitted-intensity distri bution of the narrowband filter from normal incidence to 60° oblique incidence. The simulation and the experiment results show that it has more accurate result and higher efficiency than the traditional beam spatial superposition method.