Infrared smoke plays a vital role in battlefield photoelectric countermeasures. Transmittance and effective shielding area are crucial indicators in the evaluation of infrared smokescreen shielding performance. The traditional evaluation technique has drawbacks of lacking precision, time consumption, and low automation level. Therefore, an efficient and accurate technique is urgently desired for the evaluation of infrared smokescreen shielding performance in the battlefield environment. We propose a technique based on the grayscale value for evaluating infrared smokescreen shielding performance. First, a description of the field test system is introduced. Second, the transmittance and effective shielding area mathematical models are deduced and constructed. In particular, we discuss the assignment methods of the background and smokescreen grayscale value in the transmittance mathematical model. Finally, the evaluation software is designed, and a field test sample calculation is included to illustrate the procedure of the proposed technique. The results prove to be of enough precision for engineering applications, which is of great significance for engineering guidance.