Abstract: In order to enhance output power of a fiber laser, thulium doped fiber with large mode area was used to suppress the nonlinear effect. Optimizing parameters of a thulium-doped fiber in an inhomogeneous Bragg structure, a fiber with mode area of 719μm² was obtained under the condition of single-mode transmission. Based on this fiber, a theoretical model for large mode area thulium-doped fiber amplifiers pumped by 793nm wavelength was established. Because large mode area fiber can reduce optical power density and suppress Stokes light power, compared with the ordinary single-mode fiber amplifier, this kind of fiber amplifiers under high pump power can get higher output power. The results show that, when pump power is 100W, compared with the conventional single-mode fiber, the conversion efficiency increases 5% and reaches 40%, and the output power reaches 41.01W. The research is of great value for design of actual thulium-doped fiber amplifiers.