Generally, for gas imaging, narrowing the response of the camera sensor around the target gas’s absorption band would increase the imaging contrast. In this paper, A filter-free narrowband metallic cavity quantum well infrared photodetector is proposed. The metallic cavity is formed by Ti/Au film coating on the detector’s mesa. The geometry of the cavity is properly designed to sustain cavity mode resonating at 10.6 μm. With strong resonance, the absorption efficiency of the embedded quantum well active layer maintains high level (~74%) even with a fairly low doping concentration (~1×1017cm-3). And the bandwidth is as narrow as 0.22 μm. A waveguide model is presented and used to analyze the metal cavity quantum well infrared photodetector, and it is found that the metal film coating on the side wall played an important role in enhancing the resonance and narrowing the spectral line width.