The double-clad ytterbium-doped fibers are the common gain material of high power fiber lasers. Because of the small core size, it is easy to arouse the stimulated Brillouin scattering and stimulated Raman scattering effects in the fiber resonator. Once the stimulated Brillouin scattering and stimulated Raman scattering are taking place, the high intensity signals become the major noise source and play an important role on the lasers output characters. Experimental study on them is reported in this paper. The results are shown that the pump way, the output mirror reflectivity and stimulated Rayleigh scattering play important roles to them in the fiber laser resonators, especially stimulated Rayleigh scattering. Stimulated Rayleigh scattering provides additional feedback to fiber resonators, not only narrow the laser linewidth, but also make the threshold of stimulated Brillouin scattering decrease rapidly.