A method for predicting MRTD (Minimum Resolvable Temperature Difference) based on the sampled IR imaging system digital simulation was proposed. Standard four periodical rectangular patterns with typical spatial frequency and radiometric temperature were generated according to signal transfer equation. Then, system geometric distortion, blur effects, noise characters and nonlinear effects were simulated with modulation transfer function method and pixel processing method, respectively. Finally, MRTD prediction was accomplished according to the statistical discrimination of observers to simulated rectangular pattern. Simulation results show that there is a good agreement between MRTD prediction and MRTD experimental measure.