In view of the problems such as the mismatch of the structure and parameters of the threshing device and the high moisture content of the corn kernels during direct harvesting in the hilly area of the south, the key components of the threshing device were analyzed theoretically first, and then the main structural parameters of the corn threshing device were determined. Based on this design, a flexible threshing device composed of flexible arc-shaped bar teeth and round steel bar concave plate screen was created. The optimal combination parameters of the device that affected the corn kernel breakage rate and loss rate were found out through single-factor experiments and three-factor three-level response surface experiment, and the optimal combination parameters were used for field verification experiments. The results showed that the device could effectively reduce the corn kernel breakage rate. Compared with the rigid threshing device, the corn kernel breakage rate was reduced by 12.08%. The optimal combination parameters that affected the corn kernel breakage rate and loss rate inclued the drum speed of 1 027 r/min, the threshing gap of 33 mm and 66 barbs. With these conditions, the kernel breakage rate and loss rate were 3.49% and 2.56%, respectively. In the field verification experiment, the kernel breakage rate and loss rate were 3.61% and 2.78%, respectively, and all indicators met the national standards.