Various factors contributing to the nonlinearity response in infrared focal plane array (IRFPA) were analyzed. Then based on the dependence of quantum efficiency on the wavelength of incident photon flux, the global nonlinearity in the response of HgCdTe IRFPA was calculated under quasi ideal conditions. In consideration of nonuniformity correction, the nonlinearity in the calibrated interval was further calculated. At last, a practical case of residual nonuniformity error attribued to the nonuniformity of response nonlinearity was introduced and interpreted.