Pointing to the deficiency of the in-situ test measurement of the thermal infrared imager,a set of portable test device of the thermal infrared imager with MRTD has been designed.First,test the MRTD of the thermal infrared imager in-situ.And then,calculate the apparent temperature difference of the target through the model of the atmosphere trasmissivity.At last,through the Johnson criterion,analyze,calculate and obtain the ability of detecting and recognizing the target by the thermal infrared imager.The experiment result shows that the method is exactly feasible and may be used to test the findable distance of all rinds of the thermal infrared imager.