The invention relates to a surface defect automatic detecting device for production process of a section bar rolling mill production line, which comprises a fixing device, a control device, video cameras, an image collecting computer, an information processing server, a light-emitting diode (LED) light source, a local area network, a process control network system, a spraying device, a pinch roll, a conveying roller way, stripper rubber and compressed air. The surface defect automatic detecting device for production process of the section bar rolling mill production line is characterized in that the number of the video cameras is determined according to the maximum number of the video cameras needed in the most complex surfaces of all varieties of the section bars to be detected. The surface defect automatic detecting device achieves automatic detection of surface defect of section bars (including varieties of heavy rails, H-shaped steel, joint steel, channel steel, steel angle and the like) by controlling and adjusting focal distance and angle of the video camera and the number of the video cameras. The surface defect automatic detecting device for production process of the section bar rolling mill production line is strong in adaptability, the same configuration can be applied to heavy rail processing lines and profile steel finishing lines, and accordingly software, hardware and spare parts of the device are universal, and manufacture and maintenance cost of equipment can be saved.