HighlightsThis research was aimed at assessing the performance of a head-feeding combine harvester.The rotary grate concave threshing and separating unit were effective for improving and preventing the threshing cylinder from being jammed and improving working efficiency.More broken grains were obtained at a faster speed of concave grate.The speed of feeding chain was significant on the performance parameters that were evaluated.Abstract. It is easy for the threshing cylinder to get blocked in the head-feeding combine harvester with the fixed grate concave threshing and separating unit to affect the working efficiency when high yield rice is harvested. The rotary grate concave threshing and separating unit was designed to solve the above problem. The rotary grate concave surface revolved in the same direction along the cylinder arc of threshing drum. A test-bench of the rotary grate concave threshing and separating unit was developed. The rotational speeds of cylinder (RC), the speed of the concave grate (SG), and the speed of feeding chain (SF) were regarded as experimental factors, and the grain loss rate, broken grain rate, and impurities rate were selected as indices. Central composite design combined with response surface methodology and multi-objective optimization method were employed to conduct experiments and explore the interaction effects of factors on indices. The optimal operating parameters were nonlinearly optimized and determined to be RC of 547.6 rpm, SG of 0.93 m·s-1, and SF of 1.06 m·s-1. Under these conditions, the predicted values of the grain loss rate, broken grain rate, and impurities rate were 1.95%, 0.29%, and 0.58%, respectively. Verification experiments indicated that the observed values were in agreement with the predicted values. Results suggested that the rotary grate concave threshing and separating unit developed in this study were effective for preventing the threshing drum from being blocked, and there was a good potential for performance improvement of head-feeding combine harvester. Keywords: Central composite design, Cylinder speed, Feeding chain speed, Head-feeding combine harvester, Response surface methodology, Rotary grate concave, Threshing and separating unit.