In order to improve the problems of low contrast, lack of detailed information, and poor fusion effect of the fusion image, a new algorithm for infrared and low light image fusion, that is, a fusion algorithm based on Cauchy fuzzy function under Tetrolet transform, is proposed. First, the source image is decomposed into two sets of high-frequency and low-frequency coefficients by Tetrolet transform; then, a regional energy-Cauchy fuzzy function rule is proposed for the low-frequency coefficients and the regional Laplacian energy sum is used for high-frequency coefficients; finally, the inverse Tetrolet transform is used to transform the new high-frequency and low frequency coefficients into the final fusion results. Experimental results show that the proposed method has certain advantages in improving the contrast of fused images and preserving image details.