A high-power all-fiber mid-infrared supercontinuum laser source that consists of a 1.55 μm nanosecond-pulsed erbium-doped fiber laser, a cladding-pumped thulium-doped fiber amplifier and a segment of single-mode ZBLAN fiber is reported. Firstly, the pulse breaks up through modulation instability and the laser shifts to a longer wavelength through Raman processes in the single-mode fiber. Secondly, the nanosecond pulses are amplified through a thulium-doped fiber amplifier. Finally, the output spectrum from the thulium-doped fiber amplifier is extended to the mid-infrared wavelength by using a segment of single-mode ZBLAN fiber. The output power from the ZBLAN fiber is 2.2 W with the launched pump power of 3.95 W at 2.0 μm wavelength region, the emission spans the wavelength range from 1.9 μm to 3.75 μm with a 10 dB bandwidth of greater than 1600 nm. In addition, a higher output power mid-infrared supercontinuum laser source is achieved by increasing the average output power of the thulium-doped fiber amplifier. The ZBLAN fiber produces an average output power of 16.2 W with the coupling pump power of 21 W at 2.0 μm wavelength region, the emission spans the wavelength range from 1.9 μm to 3.5 μm.