In this paper, we developed an auto-focus image quality evaluation function from the histogram feature function (HFF) of a high-speed camera. Based on this function, we then proposed a fast auto-focus search algorithm. We further demonstrated that the auto-focus image quality evaluation function we proposed requires fewer computational resources, while providing a more effective focusing area and more moderate sensitivity, compared to the traditional evaluation function. In contrast to the traditional climbing method, our proposed fast auto-focusing search algorithm also can spot the focus point in a shorter time, effectively reducing the round trips of the focusing motor and improving the focusing accuracy.