The format of infrared focal plane arrays (IRFPAs) is continuously increasing, and the spatial non-uniformity in the irradiance affects the result of large-format IRFPAs’ responsivity measurement. When placed in the test system, different pixels on the IRFPA receive different radiation flux due to the specific geometric setup of the dewar, resulting signal response uniformity. This is not the inherent characteristics of the IRFPA and thus should be corrected. The existing methods of correcting irradiance spatial non-uniformity simply consider the solid angle subtended by the aperture of the cold shield viewing from the center of a pixel. However, these methods are only applicable for the case of an infinite blackbody behind the cold shield. This paper presents a more comprehensive and appropriate correction, taking account of the specific geometric setup of the dewar, especially a dewar window of finite size. The relative irradiance received by different pixels on an IRFPA, as a correction factor, is simulated directly through matrix calculations. We apply the FOV correction profile to a 640 x 512 HgCdTe IRFPA with a pitch of 25μm, finding that responsivity ratio of the peripheral pixels to the central pixels on the IRFPA has changed from an original value of 88.0% to 96.4% after correction. This method has been proven to be useful and effective in obtaining more accurate description of the IRFPA performance for further analysis.