The two-point nonuniformity correction algorithm for IRFPA is theoretically and perfectly deduced based on Planck Law and the linear response model of infrared detector. The physical motivation of the two-point nonuniformity correction is hereby disserted for the first time. It is theoretically proved that the algorithm is perfect for nonuniformity correction provided that the response of IRFPA is linear and stable. The residual nonuniformity is initiatively analyzed.