Abstract: Based on the full study for the under-sampling aliased response of focal plane thermal imaging systems, the Aliasing As Noise(AAN) method and the optical imaging system under-sampling evaluation method on the basis of information theory were analyzed. The under-sampling aliased response was regarded as a system noise and put into MRTD model like other system noise. The new MRTD model containing the under-sampling noise was obtained. By contrasting the data which got from the simulation of the new MRTD model and references, the change between the simulation and data in references is basically same. So the new model can be used to evaluate the impact of under-sampling on the performance of thermal imaging systems, however it needs further amendment for more accurate. The next step, the new MRTD model with aliasing response will be amended by measured MRTD data of focal plane thermal imaging systems.