When the infrared imaging system measuring the radiation characteristics of the target, the internal stray radiation is an important factor affecting the measurement results, and the environmental temperature will directly lead to the change of the internal stray radiation. This paper analyzes the phenomenon of the gray drift of the system output caused by the environmental temperature change, and derives the functional relationship between the drift amount and the ambient temperature. A simulation analysis model of internal stray radiation was established to verify the theoretical analysis. Copyright © 2021 SPIE.