Definition of infrared FPA linearity,calculation and test principle are introduced.Linearity of a middle wave infrared FPA detector was tested.Test error of the linearity was analyzed.The test results can provide theoretical and experimental foundations for measuring the linearity of infrared FPA detector.The experiments prove that the error of blackbody′s emissivity is the lethal factor in linearity measurement.The standard deviation of a middle wave infrared FPA′s linearity was tested and it is less than 2%.