In this paper the qulse pumped Yb-doped double-clad fiber laser is studied experimentally. Pumped by 100 μs long pulses, which repetition rate ranged from 2 kHz to 8 kHz, the fiber laser outputs shorter than 10ns stable pulses. Further the dynamics of the output character of the fiber laser is analyzed. For the first time, the authors provide that the theory of the co-operation of Rayleigh scattering (RS) and stimulated Brillouin scattering (SBS), which results in ns pulses output from the pulse pumped fiber laser.