Autofocus technology is a crucial part in the detection of thin film transistor liquid crystal display (TFT-LCD). Because the fast and accurate autofocus is required, an autofocus microscope system is adopted to detect TFT-LCD. The digital grating is projected onto the surface of the measured object. The reflected light is split into two paths using a beam-splitting prism. One is imaged onto the area-array charge-coupled device (CCD) directly, whereas the other is split into two paths using another beam-splitting prism again. A reflecting mirror is used to couple two images of the digital grating onto a line-array CCD. The defocusing direction and defocusing amount are judged by comparing the sharpness of two grating images on the line-array CCD. In addition, a micro-displacement platform is used to move the objective lens and achieve the microscope autofocus. Thus, this study analyzes the digital grating, which affects autofocus, and uses a variable-period digital grating for focusing.