Gas laser based on hollow-core fiber (HCF) is an effective means to realize mid-infrared laser output. Generally, according to the transition selection rule, one pump absorption line corresponds to two lasing transition lines. A single-pass HCF HBr laser with a 4.3 μm single spectral line is realized by means of air pressure control. Using the self-developed 1958 nm continuous wave high-power narrow linewidth thulium-doped fiber amplifier as the pump source, a 5-meter-long anti-resonant HCF filled with low-pressure HBr gas is pumped. The laser output with 4.3 μm single spectral line of isotope H ⁷⁹Br and H ⁸¹Br is realized respectively through air pressure control, the maximum laser power is 350 mW, and the total optical-optical conversion efficiency is about 8%. The output laser spot is measured by the self-built optical fiber scanning device, and the result shows that it is a fundamental mode.