To solve the problem of excessive enhancement when a POSHE algorithm is used to enhance infrared images, a histogram transform method based on 'optimal gray-level distribution' fitting is put forward and is verified in a MATLAB simulation experiment. The result shows that the method can not only alleviate the problem of excessive enhancement and eliminate the block effect phenomenon due to processing by the POSHE algorithm, but also inhibit the 'cold reflection image' of a cooled infrared detector. The whole information of an infrared image can be retained and form good visual effectiveness while the contrast of the image is enhanced.