Auto focusing technology based on image processing is the main direction of auto focusing, in which the image sharpness evaluation function is the key to the success of focusing. The imaging details of infrared images are less than those of visible images, the unbiased and single peak characteristics of the simple image sharpness evaluation function are discounted, so the traditional auto focusing method is not suitable to the infrared images. A two segment fast searching auto focusing method is proposed based on the combination of coarse and fine auto focusing methods. The method divides the auto focusing into two steps. First, the focal position is found quickly by coarse tuning. And second, the precise position of the focal point is finally found by fine tuning. Experiments show that this method can automatically focus the infrared imaging system, and the system has good stability and real-time characteristics.