The photocurrent spectra of metallic cavity quantum well infrared photodetectors under oblique incidence have been measured with incident plane being perpendicular to or parallel to the long axis of the device. The dependence of the resonant mode of the metallic cavity on the incident angle is investigated experimentally and theoretically. The experimental results show that the resonant wavelength of the cavity mode does not change with the incident angle when the incident plane is perpendicular to the long axis of the device, while it moves to shorter wavelength with the increase of the incident angle when the incident plane is parallel to the long axis of the device. The measured results are in good agreement with the calculated results of the derived formula which describes the relationship of the resonant wavelength and the incident angle. The angle dependences of the Fabry-Pérot filter and the plasmonic filter are also analyzed and compared. The results show that the metal cavity quantum well infrared detector has relatively good angle robustness, and it is expected to develop a quantum well infrared photodetector whose response wavelength is completely insensitive to the incident angle by using three-dimensional metal cavity.