In this Letter, a high-power 1.9-4.2 μm supercontinuum (SC) laser source with a real all-fiber structure is reported. A 12 m length of ZBLAN fluoride fiber was used as the nonlinear medium, which was pumped by a thulium-doped fiber amplifier through a firm fusion-spliced joint between silica fiber and itself. The obtained SC laser had a high spectral flatness with a maximal 10 dB bandwidth of 2090 nm spanning from 1960 to 4050 nm. A record power of 15.2 W for an all-fiber mid-infrared SC laser was measured. The average spectral power density of this SC laser was as high as 7.2 mW/nm. In particular, the SC power beyond 3.0 and 3.8 μm was 8.1 and 1.08 W which corresponded to a power ratio of 53.2% and 7.1%, respectively. This Letter, to the best of our knowledge, represents the brightest all-fiber mid-infrared SC laser, and provides a promising high-power pump light for SC generation in cascaded chalcogenide fibers.