Auto-focusing system, projector with auto-focusing system and auto-focusing method

The invention discloses an auto-focusing system, a projector with the auto-focusing system and an auto-focusing method. The auto-focusing system comprises an image sensing component and an image signal processor; when an image picture is projected by the projector, a corrected image is provided in part of the image picture, the image sensing component installed by the projector captures the corrected image, and then the image signal processor analyzes the corrected image to generate a brightness parameter value, and synchronously changes the brightness parameter value with the adjustment of the focal length of a projection lens; when the maximum brightness parameter value is found, successful focusing is determined; and the problem that the lens of the projection lens is out of focus due to thermal expansion and cold contraction after the temperature rises can be solved. 种自动对焦系统、具有自动对焦系统的投影机以及自动对焦方法，自动对焦系统包括影像感测组件、及影像信号处理器，通过投影机投影影像画面时，在部分的影像画面中设有校正图像，由投影机安装的影像感测组件来撷取校正图像，再由影像信号处理器来分析校正图像产生亮度参数值，随着调整投影镜头的焦距同步