A cladding pumped passively Q-switched Yb-doped fiber laser with Cr4+:YAG saturable absorber is reported and stable nanosecond pulse trains is gotten. Two laser configurations of linear cavity and ring cavity are set up, and both of them can effectively restrain the generation of stimulated Brillouin scattering (SBS) and the generation of self-pulse. A ring cavity with Cr4+:YAG inside the cavity to obtain stable 1 μs pulse trains is established. The amplitude fluctuation and timing jitter are below 5% (RMS). Further more, stable pulse trains by using a high reflective fiber Bragg grating (FBG) as a resonator mirror in a linear cavity is reached. The tunable pulse repetition rate is from 9.1 kHz to 30.3 kHz and the minimum pulse duration is 156 ns which is much less than microsecond pulses reported before. Although the average output power is only hundreds milli-watts now, it is convinient to boost the power with a stage of fiber amplifier because the laser uses a fiber coupler as the output of pulse trains. This kind of pulse laser can be used as a seed source and amplified to tens of watts for practical applications in the future.