The invention discloses a middle-infrared thulium-doped fiber laser amplifier which is composed of a seed optical source, an optical isolator, a focusing lens, a pumping optical source, a multi-path fiber coupler, double clad thulium-doped fiber, a collimating lens and a beam splitting plate. The position relationships of the components are as follows: the optical isolator, the focusing lens, the multi-path fiber coupler, the double clad thulium-doped fiber, the collimating lens and the beam splitting plate are sequentially arranged along the laser output direction of the seed optical source, an input end of the seed optical fiber of the multi-path fiber coupler is positioned at a focus of the collimating lens, an output end of the seed optical fiber is welded with the double clad thulium-doped fiber, the other end of the double clad thulium-doped fiber is positioned at a front focus of the collimating lens, and the beam splitting plate and the output light beam of the collimating lens are arranged at an angle of 45 degrees. The laser amplifier of the invention has good heat dissipation characteristic, improves the pumping power and can obtain high-power mid-infrared laser output.