Abstract: Abstract: Soybean is one of the most important grains and oil crops in China, due to a major source of oil and high-quality vegetable protein. It is very necessary to realize the low-loss harvest of soybean fields in the southern and Huanghuaihai regions. In this study, a 4LZ-1.5 type combine harvester was designed to consider the harvest of soybean breeding plots. The operation process was also theoretically analyzed for the soybean reel harvesting, according to the soybean pods easy to be fried at maturity. A numerical model was established for the reel structure and motion parameters. Some parameters were then optimized, such as the reel radius, reel speed ratio, and reel rotational speed. After that, the soil removal mechanism was optimized for the soybeans with the low pods and easy-to-shovel soil, according to the statistical parameters of soybean grain size. Furthermore, a soybean threshing and separation device was designed for the easy threshing and breakage of soybeans at maturity. The arrangement and distribution of the arch teeth were also determined via the increasing characteristics of the second-order arithmetic sequence. The angle of the deflector was adjusted for the various input amount. The blade fan-combined screen cleaning device was designed to optimize the structural parameters of the fan and cleaning screen. The pneumatic grain unloading device was designed to optimize the vortex fan. The plot harvesting required the high soybean mixed seeding, particularly for the breeding plot with a small planting scale and many varieties, compared with the field production. Frequent seed cleaning was required after harvesting to reduce the missed and mixed seeding, according to the harvest requirements of the breeding area. An optimization mathematical model was established to develop the seed cleaning device, together with the objective function and constraint conditions. A numerical simulation was then carried out using MATLAB, in order to determine the structural parameters of the seed-cleaning device. A series of comparative experiments in field harvesting were conducted in the Xinjiang Uygur Autonomous Region, Shandong province, and Hebei province of China. The better operating performance was achieved in the 4LZ-1.5 soybean combine harvester: the loss rate < 3.5%, broken rate < 1.5%, and impurity rate < 1%, fully meeting the soybean harvest requirements in most hilly areas. After harvest testing of 14 soybean varieties in a 3 m×6 m breeding plot, the test results were all achieved: the loss rate < 3.0%, broken rate < 1.5%, impurity rate < 1.0%, mixed seed rate < 0.2%, and the clearing time of 200-270 s, fully meeting the requirements of the harvesting operation indicators in the breeding plots. Compared with the existing soybean harvesting machinery, the harvest loss rate of 4LZ-1.5 soybean combine harvester was reduced by 1.5-5.0 percentage points, the broken rate was reduced by 3.5-6.5 percentage points, and the impurity rate was reduced by 2.0-7.0 percentage points. The findings can provide a strong reference for the structural optimization of operating parameters in soybean harvesters.