Because of the influence of conduction of heat and environment and the limitation of the infrared imaging system, the contrast of infrared image is low, and the edge is commonly. Hereinto, the typical example is that the distribution of image histogram is excessively concentrated. Algorithm of image nonlinear extension enhancement based on distributing characteristic of histogram is presented, which spreads the uneven longitudinal arranging idea of image pressing-extension digitalization into the uneven transverse extension of the image histogram. On the basis of presentation of algorithm, the enhancing effect of it is contrasted with that of histogram linear extension algorithm.