In order to realize high efficiency auto focusing function in infrared imaging system, the characteristics of infrared imaging systems are analyzed. The optimization methods of auto-focusing functions in infrared imaging systems are discussed and summarized. Combined with practical engineering applications, a method for estimating the local random fluctuation noise of the image sharpness evaluation function is proposed. The method improves the reliability of automatic focusing processes. By introducing the sensitivity of image sharpness evaluation function as feedback into the climbing process, the convergence speed of the climbing algorithm can be optimized. The optimization methods were applied to engineering applications, and its implementation verified the correctness of the design and the effectiveness of the optimization methods.