Based on an ideal slope-profile model as well as spatial convolution transform with target extension, a MTF ap- proach was proposed to solve on-orbit evaluation for thermal infrared band of sampled imaging system. Simulation results show that the absolute evaluation error of the system MTF at half normalized spatial frequency is about 0.05 for an ordinary system with good performance. Meanwhile, the system MTF characteristics of FY-2C satellite thermal infrared band (IR1) were quantitatively evaluated with observed data from 2006 to 2008, which is consistent with those measured by on ground testing. By using the derived system point spread function(PSF), the images of FY-2C IR1 were recovered with more detailed textures. It improves the monitoring performance for some typical typhoon. The ideal step-profile model used in the traditional on-orbit MTF evaluation can be regarded as a special case of the proposed ideal slope-profile one where the parameter N slope equals zero. Therefore, the proposed method is also suitable for on-orbit MTF evaluation of visible and near-infrared bands.