A new technique of histogram double equalization for the infrared thermal image is presented. At the same time of having histogram equalization, the gray redundance is removed and the gray level is reconstructed with equal interval in the whole display region. Then the continuous gray images are obtained. The problem of the large dynamic range of input signal against the small dynamic range of output display is solved in the signal processing in infrared thermal image system. Theoretical and experimental results show that the detail and definition of infrared thermal image are greatly improved.