Stimulated Brillouin scattering (SBS) is an important factor that affects the increase of output power of single-frequency fiber amplifier. The impactions of length of transmitting fiber on the SBS threshold power of continuous-wave single-frequency all fiber amplifier are experimentally studied. Two stage continuous-wave all fiber amplifier is set up with the seed line-width of 2.35 kHz. By changing the length of the transmitting fiber between the gain fiber and the output isolator, the power feedback and spectral feedback of amplifier as a function of output power are measured, and the impactions of the length of transmitting on SBS characteristics of all fiber single-frequency laser are analyzed. The experimental results match very well with the theoretically calculated results.