A high power all-fiber nanosecond-pulsed thulium-doped fiber master oscillator power amplifier (MOPA) system that consists of low power passively Q-switched thulium-doped fiber laser seed and two-stage thulium-doped double-clad all-fiber amplifiers pumped by high power multimode diodes at 790 nm is studied. The repetition rate of the thulium-doped fiber laser seeder is tunable from 77 kHz to 240 kHz with the increase of pump power, the minimum pulse width is measured to be 250 ns and the maximum average output power is 73 mW. Two-stage double-clad thulium-doped all-fiber amplifiers are used to boost average power to 20 W, corresponding to single pulse energy of 100 μJ. The pulse width is 270 ns, and laser center wavelength is 1966 nm.