Single-frequency pulsed fiber lasers have aroused intense interest due to their excellent performances in terms of good coherence, compact structure and good beam quality, which have been widely used in different areas, such as coherent LIDAR, nonlinear frequency conversion, and remote sensing. In this paper, a linearly polarized single-frequency pulsed fiber laser is reported. The narrow linewidth all-fiber oscillator is built by using an all-polarization-maintaining ring cavity structure. A section of unpumped polarization-maintaining ytterbium doped fiber is fused in the cavity as a saturated absorber to produce ultra-narrow bandwidth dynamic grating, which can be used for longitudinal mode selection. Thus, the laser can realize single-frequency operation. A 976-nm semiconductor laser is used as a pump source to generate a hybrid pump laser, which contains both pulsed component and continuous component. As a result, a long-pulse laser is achieved with a repetition rate of 10–90 kHz and a pulse duration of 1–8 μs. At the same time, the effects of pump power and repetition rate on the time domain, the frequency domain and the power characteristics of the output laser are investigated. It is found that there is an optical bistability in the frequency domain characteristic of the output laser within a certain power range. And the influence of the laser power characteristic on the longitudinal mode selection mechanism of dynamic grating is analyzed. Finally, through parameter optimization, single-frequency pulsed fiber laser is achieved with a center wavelength of 1064 nm, a linewidth of about 23.5 MHz, a repetition rate of 10–90 kHz, a pulse duration of 4–8 μs, and a polarization extinction ratio of about 29 dB.