The transmission window of reflection spectrum for phase-shift grating products is not tunable. To solve this problem, a device with Bragg grating-assisted single-mode fiber resonator is proposed. Using the signal flow graph method, the mechanism of the device is analyzed in detail. Meanwhile, analytical expressions of reflectivity and its finesse, full width at half maximum (FWHM) are derived. Matlab computation results show that reflection spectrum of the proposed device is similar to that of phase-shift grating. Location and depth of transmission window are tunable in its reflection spectrum. Influences on its finesse and FWHM by the parameters of the fiber resonator are also discussed, which provides the theoretical basis for improving its performance as frequency selection element for a narrow line width of single-frequency fiber laser.