Based on the wavelength characteristic of mismatch fiber coupler, the effects of the distance d and the mismatch Δβ between the two cores, the coupling length z on its spectrum characteristics are analyzed theoretically. It is proposed that the fiber coupler can be a comb-filter when Δβ and d are very small. Δβ mainly effects the extinction ratio, d and z mainly effect the interval of the resonant peaks respectively when the two cores are matched. An all-fiber comb-filter based on twin-corn fiber coupler is experimentally fabricated by splicing a section of single mode twin-core fiber (TCF) with core center-to-center distance of 12 μm between two single mode fibers (SMFs), and the filter′s extinction ratio and insertion loss are 25 dB and 3.1 dB respectively. The interval of the resonant peaks can be changed by using TCF of different lengths, and the peak wavelength and extinction ratio are modulated by ultraviolet light of 248 nm.