For the evaluation of target stealth camouflage effect in a single infrared image, based on the classical Structural Similarity (SSIM) algorithm, the gradient value is extracted by the improved quad-direction gradient operator, and the gradient difference function is established to replace the structural similarity function in SSIM algorithm.An image similarity evaluation algorithm based on the Improved Gradient Similarity (IGSIM) is proposed.Based on the algorithm, the experiment of determining the number of equal blocks in an infrared image and the experiment of evaluating the targets infrared stealth camouflage effect are designed.The results show that the evaluation index obtained by the algorithm is more consistent with the visual characteristics of human eye compared with that of the classical algorithms, and the algorithm can better deal with the evaluation of the targets infrared stealth camouflage effect in a single infrared image.