Given the problems of color distortion and background noise amplification using contrast limited adaptive histogram equalization (CLAHE) to enhance X-ray images of airport security inspection, this study proposes an X-ray image enhancement algorithm based on three-level image fusion and CLAHE. Specifically, the X-ray image is converted into RGB and HSV images for CLAHE enhancement respectively. The enhanced images are fused by the Euclidean norm for the first-level fusion. Then, the fused images are sharpened by unsharp masking (USM), during which the second-level image fusion is performed according to the mask. Finally, the sharpened images and the original image are combined according to the coefficient to complete the third-level fusion. The simulation experiment results show that the proposed algorithm effectively improves the contrast of security inspection X-ray images, increases the average peak signal-to-noise ratio (PSNR) by 7 dB, and suppresses the color distortion and background noise in the enhanced images. This algorithm helps to improve the accuracy of identifying prohibited items in X-ray images and thus has a positive impact on the construction of a safe airport.