Aiming at the problem that the impact strength of the threshing element on the ear is too high in the threshing process of the corn direct harvester commonly used in northern China, a corn cone threshing and separating device with low loss and high efficiency was designed. Firstly, the structure of threshing drum was analyzed, the contact model between ears and threshing concave was established, and the range of the optimal parameters of the drum was determined. Then, the threshing and separating devices with different parameter combinations were simulated and optimized by EDEM dynamics software. The results showed that the threshing effect was the best when the cone angle of threshing drum was 8°, the concave was a left-handed circular tube structure, and the rotating speed of the drum was 400 r/min. Compared with the traditional threshing drum, the impact force of threshing elements in the threshing section on the ears was reduced from 284 N to 234 N, and the impact reaction force was reduced from 202.54 N to 171.52 N, which verified that the conical threshing drum had the characteristics of reducing the impact strength on the ears. Finally, the kinematics and dynamic balance of the conical threshing device were simulated by ADAMS software, which verified the reliability of the research results and the feasibility of the theory. The unbalanced inertial force generated by the system meets the requirements of national agricultural machinery design. The research ideas and conclusions of this paper can provide a theoretical reference for the innovative design and structural improvement of the corn threshing and separating device.