Abstract: Abstract: With the rapid development of agricultural mechanization in China, the combine harvester has been applied widely in the past decade. Compared with foreign study, however, domestic studies of the vibration intensity as well as the whole anti-vibration performance of the combine harvester in the various operating conditions were still in its infancy. And few researches were focused on the vibration stability of combine harvester chassis frame, although which was the installed base of the combine harvester, not only carrying the cutter, conveyor, threshing and separating device, but also supporting the engine and other mechanism. During the combine harvester working, the exciting vibrations from the pavement, cutter, engine, vibrating screen and threshing cylinder would cause the combine harvester vibration. Once those excitation frequencies were closed to or overlapped the natural frequencies of the combine harvester chassis frame, the whole body of the combine harvester would produce resonance seriously, which brought about adverse effect on work performance, service life and reliability of the combine harvester. Therefore, in order to improve the operating performance and optimize the chassis structure of the combine harvester, it was particularly necessary to carry out the modal testing and analysis on the chassis frame of the combine harvester.In the paper, the parametric modeling of the combine harvester Julong-280 chassis frame was developed using the three-dimensional modeling software UG. After meshing, the modal parameters as well as the modal vibration shapes were calculated utilizing the NX Nastran, and the first 4 modal frequencies were 20.44, 36.35, 41.75 and 43.67 Hz，respectively. In order to verify the accuracy of the modal analysis results, the laboratory modal testing about the combine harvester JuLong-280 chassis frame was implemented by exciting vibration method with the DH5902 dynamic analysis system. Then experimental modal parameters were obtained, and the first 4 modal frequencies were 20.02, 38.82, 41.02 and 45.78 Hz，respectively. By comparing the above results, it could be seen that the theoretical modal frequencies were close to the experimental modal frequencies (the maximum error is 6.36%) and their mode shapes were also basically the same, which verified the correctness of the calculated results and the accuracy of the chassis frame finite element model.Then, the excitation frequencies of the exciting vibrations from the pavement as well as the engine, cutter bar, vibrating screen and threshing cylinder of the combine harvester were analyzed, and the result demonstrated that the natural frequencies of the combine harvester chassis frame were closed to or overlapped the above excitation frequencies, which implied the combine harvester was vulnerable to resonance in operation. Therefore, in order to improve the modal frequencies to avoid the resonance, a reasonable optimization proposal was conducted by the Altair HyperOpt method. The optimization results indicated that the first 4 modal frequencies of the improved frame increased respectively to 21.70, 36.61, 43.64 and 46.33 Hz with the weight of the frame increased by 3.77%. These values avoided the range of external excitation frequencies of the threshing cylinder 15-18.3 Hz as well as the engine 38.3-41.7 Hz, which means the improvement could avoid the resonance effectively.