We have demonstrated an 8.4μm stable single-mode quantum cascade laser using a two-section distributed Bragg reflector design. Output power of 252 mW, a threshold current density of 1.04 kA/cm2, and a characteristic temperature of 206 K were achieved during continuous-wave operation at room temperature. Robust single-mode emissions with all side-mode suppression ratios exceeding 25 dB were obtained at injection currents ranging from 420 mA to 900 mA and heat-sink temperatures ranging from 10 °C to 102 °C. Benefiting from the physical isolation of the two sections and from being mounted epilayer-side down on patterned submounts, the tiny tuning coefficients of the device are −0.003014 cm−1mA−1 and −0.0762 cm−1K−1. This type of single-mode quantum cascade laser with fine tuning characteristics is required to perform accurate detection of trace gases.