Reversible lasing performance degradation is investigated on an uncoated actively- biased GaAs/Al0.45Ga0.55As mid-infrared quantum cascade laser (MIR-QCL) facet. The surface temperature rises (ΔT) on the MIR QCL are characterized before and after the device undergoes an accelerated aging burn-in test, followed by hydrogen plasma treatment. The data is visualized by spatially resolved time-domain thermoreflectance (SR-TDTR) microscopy. On the laser facet, ΔT decreases with a drop in lasing performance and ΔT increases with an increase in lasing performance. Along the laser cavity, the thermal property change is negligible before and after the aging test, independent of the optical performance change. The results verify that thermal-induced facet oxidation is the main reason for the rapid degradation of the lasing performance in the early aging stage.