In order to improve the output performance of a high-power Thulium-Doped Fiber Laser (TDFL) and increase the optical-optical conversion efficiency of the system, a high-power TDFL with an all-fiber Main Oscillation Power Amplification (MOPA) structure was developed, which can operate in both Continuous Wave (CW) and Quasi-Continuous Wave (QCW) modes. First, a laser oscillator was built to study the output characteristics of the seed source laser. Then, a thulium-doped fiber amplifier was built and connected to the laser oscillator to study the output characteristics of the MOPA structured fiber laser. Finally, the pulse characteristics of the MOPA structured fiber laser were analyzed under the QCW modulation mode. The laser oscillator achieved a continuous and stable laser output with a central wavelength of 1940 nm, and the highest average output power was 18.56 W. The slope efficiency was 54.84%, and the spectrum was free of Raman components. Using this low-power continuous laser as the seed source through the homemade thulium-doped fiber amplifier, the average output power could reach 66.9 W, and the slope efficiency was 48.48%. When the system operated in the QCW mode, the frequency and duty cycle can be adjusted, and the peak power was calculated to be 80.3 W when the frequency was 75 Hz and the duty cycle was 10%. This research is of referential significance for the development of higher power MOPA lasers in the 2 μm band.