To meet the requirement of high speed and accurate performance test of large TFT-LCD(the fifth to the eighth generation LCD),fast auto-focus method is studied.A clarity evaluation function and an auto-focus method used to inspect the glass by microscope are presented.It used a laser and a black /white CMOS camera as an auxiliary means.The CMOS camera is used for getting image of the laser spot.Evaluation criteria of definition can be acquired by calculating the transparent rate of the CMOS camera.The focus of CCD camera can be obtained by the clarity evaluation function.On this basis,how to adjust the distance between the microscope lens and the glass in order to achieve focus is determined by the position of the spot centroid of the image in the spot centroid table.Experimental results show that when the objective lens with magnification of 5 to 50 times of depth of field are used for test,auto-focus time is 0.4 seconds or less and repetitive focusing accuracy can be less than 0.1 times to 0.5 times when the glass is in the focal plane up and down within ±50 μm.