Narrow linewidth fiber lasers have lots of applications in fiber communication, fiber sensors, coherent detection and combination because of its narrow linewidth and low noise. According to the transfer matrix method, we simulated the transmission spectrum of single phase-shifted fiber grating by Matlab, analyzed the transmission spectrum of single phase-shifted fiber grating with the different phase-shifted magnitudes,locations, fiber grating lengths and the different refractive indicators of the core, and got the main factors affecting the transmittance, linewidth and position of the phase-shifted peak.Furthermore,we proposed a scheme of tunable narrow linewidth erbium-doped fiber laser using a phase-shifted fiber grating with a π phase-shifted and a location of phase-shifted at the midpoint as the filter window.