Experimental measurements on the phenomenon of pulse distortion which is induced by the stimulated Brillouin scattering (SBS) in the ytterbium-doped double-clad fiber amplifiers with the single-frequency pulse amplification are presented. In the experiment,the 14 m double-clad single-frequency amplifier with the inner-clad mode diameter 130 μm and the fiber core mode diameter 6.5 μm is adopted. The backward stimulated Brillouin scattering (SBS) pulse is observed when the peak power is up to 3.3 W in the single-frequency pulse amplification with the pulse duration of 200 ns. The pulse width of SBS pulse is apparently narrower than that of the input signal pulse. The stochastic aspect of pulse distortion and power property induced by SBS are analyzed. Meantime the frequency shift of SBS is verified using the F-P interferometer with free spectral range of 30 GHz.