Two narrow-linewidth filters, consisting of an ultra-low-reflectivity fiber Bragg grating (LRFBG) and one (or two) 2×2 fiber couplers, are designed for single-longitudinal-mode fiber lasers. The reflectivity expressions of the two filters are deduced theoretically. The characteristics of the two filters, such as reflectivity, narrow linewidth and group delay, are numerically analyzed and experimentally verified. The two fiber filters have the characteristics of high reflectivity, ultra-narrow linewidth, large group delay and good slow-light effect. What’s more, in the case of the double-ring filter, the amplitudes of resonant peaks are modulated because of the Vernier effect. Therefore, the effective free spectral range (FSR) and side mode suppression ratio (SMSR) of the double-ring filter are larger than those of the single-ring filter, which means the double-ring filter is hardly mode hopping. Because of the advantages of narrow linewidth, compact structure and low cost, both of the designed filters can be used for fiber laser as the cavity mirror to narrow the linewidth of laser further. And the results show the linewidth of the laser with double-ring filter is narrower than that of the laser with single-ring filter.