A method for predicting the minimum resolvable temperature difference (MRTD) of an infrared imaging system is proposed. The method is based on the linear system theory. It uses the modulation transfer function to establish an infrared imaging system model for simulating and generating standard four-bar patterns with different spatial frequency (it has an aspect ratio of 7:1). By analyzing the two-dimensional (horizontal and vertical) imaging quality of the four-bar pattern and introducing a contrast correction factor, the temperature difference between the four-bar pattern and the background output from the system is acquired. Then, the threshold input contrast transference is used to obtain the MRTD of the system. The simulation results show that the MRTD predicted by this method is coincident with the actual measurement value.