How to quickly and accurately realize the focusing is always the main researching content of the auto-focus-ing technology. The key step of the focusing algorithm is to select the correct evaluation function. The traditional focus-ing evaluation function has some problems in the application,especially in deep defocus conditions. In the depth of de-focus condition, the following defects are encountered:focusing failure, inaccuracy focusing in effective region and so on. To solve these problems, an improved focusing algorithm, a secondary focusing based on image processing, is in-troduced based on the existing focusing algorithm. The improved focusing algorithm can achieve stability, rapidity and accuracy of auto-focusing easily. The first focusing uses improved automatic threshold algorithm of variance function to complete the fast focusing with selecting parts of image data in the center area. The second focus selects the first part of inverted T shaped area image data, using the form of Robert gradient threshold to get precise focusing.The testing results show that the improved secondary auto-focusing algorithm has some advantages in terms of stability, speed and accuracy of focusing,especially the focusing effect is more significant when it’s in the depth of defocus.%如何快速、准确地实现聚焦一直是自动聚焦技术的主要研究内容，其中关键问题是选取正确的评价函数。传统聚焦评价函数在应用于深度离焦情况时存在一些诸如聚焦失败、有效聚焦区域内精度不够等问题。针对这些问题，为了提高自动聚焦的稳定性、快速性、准确性，阐述了一种在现有聚焦算法的基础上改进的基于图像处理的二次聚焦的算法。第一次聚焦选取中心范围内部分区域的图像数据，采用改进的自动阈值方差函数算法来完成快速聚焦。第二次聚焦选取包含第一部分的倒T字形区域图像数据，采用Robert梯度-阈值形式进行精确聚焦。验证结果表明，改进的二次自动聚焦算法在稳定性、速度和精确度方面都有较好的效果，特别是在深度离焦时聚焦效果最为显著。
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