Abstract: Abstract: As key parts of a corn and wheat combine harvester, the threshing cylinder is mainly processed with the method of welding. The processing quality of threshing cylinders, including cylindricity and verticality of outer radial discs fixed outside of threshing cylinders, is hard to guarantee. The major problems that the threshing cylinder end faces are radial run-out, which have seriously restricted the effect of threshing and selecting corn or wheat from millet straw. Thus, without online diagnosis method and system, the threshing cylinder of corn and wheat combine harvester will get poor quality. In this paper, the welding quality online detection method based on laser sensors was studied, and the online detection system of cylinder roundness and outer radial disc verticality during the welding was developed, including the threshing cylinder rotation driving system, laser sensor distance detection system, sliding guide rail system and data acquisition and display system. As to cylindricity detection, sliding guide rail that was parallel to the threshing cylinder axis was fixed on the welding platform, a laser distance detection sensor was fixed on the sliding guide rail, and a servo motor was connected to the threshing cylinder axis. While detecting, the threshing cylinder was driven to rotate, and at the same time, the laser sensor was driven to move along the sliding guide rail; and the vertical distance from the sensor to the threshing cylinder was detected, which was collected in real time by the computer. According to the threshing cylinder rotation data and laser sensor sliding track data, the threshing cylinder shape and cylindricity were calculated. As to the inspection of outer radial disc verticality, vertical sliding guide was fixed to the parallel sliding guide rail that was perpendicular to the sliding rail. Install laser sensor on the vertical sliding guide rail and keep the laser beam parallel to threshing cylinder axis. When the threshing cylinder was driven by the motor to rotate, the distance from the laser sensor to the outer radial disc was measured by the laser sensor, and the verticality of the outer radial disc was calculated by the computer. According to the method mentioned above, the online test platform was designed, and online detection was realized through data processing. Experiments were done and data was analyzed, including comparing the accuracy of the online detection system in this article and Cylindricity Measuring Instrument Talyrond130 (Taylor Hobson, England) and Laser verticality measuring instrument YHL-800 (ACROBEAM Co. Ltd, Xi’an, China), and the relative errors of duplicate tests were done by the online detection system. Results showed that, compared with Cylindricity Measuring Instrument Talyrond130, whose taper error was 0.00025 μm/mm, the absolute error of cylinder roundness detection was 0.059 mm; and compared with Laser verticality measuring instrument YHL-800, whose accuracy was 1/20 000 mm, the absolute error of outer radial disc perpendicularity detection was 0.023 mm. After 6 duplicate tests, the maximum relative error of cylindricity testing was 2.17%, the standard deviation was 0.021 mm, and the maximum relative error of perpendicularity detection was 1.93%, and the standard deviation was 0.011mm, which met the needs of the online detection of the threshing cylinder welding quality, and the efficiency of online detection system was 20 times that of traditional method. The development of the system can ensure the welding quality of large-scale harvester threshing cylinder.