Abstract: Abstract: Operating parameters such as threshing cylinder speed, concave clearance and bar spacing are the most important factors that influence the power consumption of threshing apparatus and grain loss ratio during the threshing process. In order to obtain rice threshing optimal operating parameters, investigations were conducted on the longitudinal axial threshing apparatus test bench of combine harvester. The threshing power consumption and grain loss ratio of longitudinal axial threshing apparatus were measured at the different combination of cylinder speed, concave clearance and bar spacing. Rice threshing experiment results were analyzed by carrying out regression analysis and the significance of regression effects assessing. The confidence interval of the optimal operating parameters was used to control the performance of threshing and separation process, and the optimal variables of the longitudinal axial threshing apparatus were predicted in advance and tested soon afterwards. When the total grain loss ratio was controlled within less than 0.33% and the threshing power consumption was less than 46.36 kW with a confidence of 0.95, the confidence interval of the best cylinder speed was 772.61-905.74 r/min, the best confidence interval of concave clearance was 22.18-37.93 mm, and the best confidence interval of bar spacing was 104.96-170.17 mm. The results showed that the best threshing cylinder speed was 839 r/min, the best concave clearance was 30 mm and the best bar spacing was 138 mm. These operating parameters can reduce the energy requirements of threshing process and also can lead to low grain loss.