Infrared radiometric calibration is one of the basic technologies to obtain the infrared radiation characteristics of the target. The accuracy of radiation calibration has an important influence on the subsequent target's radiation characteristic measurement. Aiming at the selection of the pixel size of the detection target during the mid-wave infrared radiation calibration, the influence on the linearity of the system during the radiation calibration is studied. Based on 640×520 mid-wave focal plane array, the central pixel, 100×100 central area, 200×200 central area and the whole target area are selected for research. In the high temperature section, the random error in the 20×20 area is 5.27%, and the minimum error in the center pixel 100×100 area is 0.01%. In the low temperature section, the random error of the 20×20 area is 1.78%, and the minimum error in the center pixel 100×100 area is 0.45%. In a certain target area, as the selected target surface increases, the error gradually decreases. The research results have certain engineering application value for the selection of infrared detector radiation calibration target surface.