A digital system of edge detecting based on a single FPGA is designed.By analyzing the signal of CCD output and the grads of grey scale breaks at the edge of picture the pixel edge of picture,the gauss filter is used to remove the noise,the pixel edge of picture is detected by the edge algorithm which can be used to detect the edge of picture automatically,and the least-squares polynomial fitting algorithm is used to detect the sub-pixel edge of picture.FPGA device is used as the core of the system and the carrier of digital circuit.The system is designed with VHDL and image input in Quartus Ⅱ 7.2 software.It is proved that the resolution nearly achieves 1/60 pixel width and can be used in the development of photoelectrical autocollimation.