An autofocus method based on the analog video signal of the microscope CCD (Charge Coupled Device)is presented, which can improve the autofocus range. In many autofocus modes, the high-frequency energy of the video spectrum is taken into account, while ignoring the changes of the low-frequency energy of the video spectrum in the focusing process. In this presentation, the spectrum of the analog video signal was analyzed, including the affection of the different frequency range analog signal on the focusing evaluation curves. Based on the analysis, we obtain the proper frequency range that fit the coarse/fine adjustment of the autofocus process. The resulting system has a wider focusing range while remaining the high accuracy. Experimental results show that the proper filter can extend the autofocus range.Using the 40× microscope objective, the effective autofocus range of the microscope can reach ±500 μm.