The invention relates to an online automatic detection system of the local straight degree of a circular-section workpiece, belonging to the technical field of mechanical processing and comprising a detection device, a lower computer part and an upper computer part, wherein the lower computer part comprises a logic control module and a data acquisition module, and the upper computer part comprises an upper computer processor, a communication module, a data processing module and a human-computer interface. Each detection section of the detection system only uses a displacement transducer, and the position of the transducer and the workpiece keep following-up. Combined with a data processing technology, the detection system is simple and reliable and solves the problem that the detection isnot accurate when the workpiece is jumpy in the radial direction. The system and a detection method thereof can be used for the online automatic detection of the local straight degree of circular-section workpieces with large length-diameter ratio, such as steel pipes, rods, shafts, stepped shafts, and the like.