Abstract: This invention relates to an automatic detection method and apparatus for tuning the frequency and phase of displaying clock of a display to match the frequency and phase of pixel clock of a PC's display interface card. Based on the synchronized displaying clock, the image shown by digital display will be stable and bright in color. The automatic detection apparatus of invention includes a clock generation unit, a sampling unit, a data processing unit, an accumulation unit, and a decision unit. The clock generation unit creates a plurality of sampling clocks and according to these sampling packet sequences, the sampling unit samples and holds the pixel signals of image frames based on the pixel clock of display interface card, and then stores these data in its registers.