The aperture shapes of cold shields have direct effects on stray light suppressing in infrared systems. The concept of photometric inefficiency is introduced to evaluate the cold shield shapes′ ability of suppressing stray light. A creative idea and algorithm of using the superposition integral method to accurately calculate the photometric inefficiency for all shapes of cold shields are proposed. Based on the method, a mathematical model is built. A typical 288×4 thermal imager is chosen as an instance to be calculated by the program. Compared with the results calculated by the ray tracing statistical method, it is concluded that superposition integral method has accordant results to the ray tracing statistical method, which has far higher speed of computation than ray tracing statistical method when they get the same accuracy.