Aiming at the problem that the stray radiation in an infrared simulation system may affect the temperature uniformity on the simulated image plane, a conception of temperature difference on an image plane is proposed and a method for calculating the maximum temperature difference on an image plane is given. A simplified infrared simulation system model for calculating the maximum temperature difference on an image plane is established. A partial irradiance deviation factor which can reveal the change of temperature on the image plane is defined. The expression for the above factor and maximum temperature difference on an image plane is deduced. The common steps for calculating the maximum temperature difference on an image plane is also given. The calculation method is demonstrated in a practical infrared simulation system and is verified in an experiment. The results show that the measured temperature differences are all within the theoretical numerical range. So, the correctness and effectiveness of the method are verified.