Mechanical threshing plays an important role in the modern soybean production, which is one of key factors causing damage of soybean seeds at the same time. It can not only bring the waste of soybean seeds and economic loss, but also affect seeds germination percentages and soybean yields tremendously, especially for mechanized precision sowing technique. Facing on the problems of higher soybean damage rate and lower soybean threshing rate, the overall design and comprehensive threshing principle of double feeding rollers and combined threshing cylinder soybean breeding thresher was proposed, critical components of combined threshing cylinder, feeding device and transmission system, etc. were designed, and the prototype was developed. Combined threshing cylinder was composed of spiral arrangement of spike tooth, bow tooth, tooth and concave plate screen. The feeding device was mainly composed of double feeding roller. Pneumatic cleaning device as mainly composed of vibrating screen and fan. The main soybean varieties from Agricultural College of Shenyang Agricultural University were selected as test materials, through orthogonal experiment analysis, the three parameters: feeding gear, drum rotating speed and concave plate clearance were selected as experimental factors. Soybean threshing rate and soybean damage rate were selected as experimental indexes. The structure and working parameter of threshing performance was optimized. Experimental results indicated that feeding gear, drum rotating speed and concave plate clearance all had significant impact on soybean threshing rate and soybean damage rate. The most optimum combination region of the moisture content was 16%～18%，the feeding gear was mid gear（222r/min）, drum rotating speed was 500r/min, the concave plate clearance was 40mm, the soybean threshed rate was 98.4% and the soybean damaged rate was 1.4%.