With the development of study on single-frequency fiber laser and amplifier, higher output power can be received. Because stimulated Brillouin scattering (SBS) effect has a great constrain on output power of single-frequency fiber laser and amplifier, so the study of infuence factor and suppression method of SBS is necessary. Based on the rate equations of thulium (Tm3 + ) ions and double-cladding fiber amplifier with SBS, a theoretical model of the single-frequency fiber amplifier is created, and the energy distribution and output power of the Tm3 +-doped fiber amplifiers are calculated. Then the effects of fiber length, pump power, Tm3 + doped concentration on output power and SBS effect of the single-frequency fiber amplifiers are discussed. Finally, it can be concluded that how to improve the output power of the fiber amplifiers and effectively suppress the SBS effect at the same time. A all-fiber Tm3 +-doped fiber amplifier is presented. The center wavelength of the Tm3 +-doped fiber laser is 1941 nm. The signal to noise ratio is 60 dB. When the pump power of the Tm3+-doped fiber amplifier is 2.15 W, the maximum output power is 0.766 W.