The temporal and spectral characteristic of a Yb doped double clad fiber laser has been studied. The influences of the pumped power and the cavity losses on the laser are investigated. It is found that the self pulsing and the nonlinear phenomena shch as Brillouin and Raman scatterings can be observed in a one mirror cavity, and the system is stabilized in a two mirrors cavity where Brillouin and Raman scatterings are efficiently suppressed. We also give a theoretical explanation about the phenomenon in our experiments.