In order to solve the high loss and blockage problem of full-feeding combine harvester harvesting green stem and leafy rice which is difficult to thresh, a tangential-horizontal-horizontal threshing and separating device was developed. On the test bench of tangential-horizontal-horizontal threshing and separating device, the parameters of axial distance of concave fences, the number of cover and axis spacing of cylinders were preferred by using comparison test. The experiments showed that the preferred parameters were the axial distance of concave fences of 10mm, 16mm and 16mm, the number of cover of 4 and 4, and the axis spacing of cylinders of 645mm and 667.5mm. Under the preferred structural parameters, threshing and separating performance experiments using different feed rates, concave clearances and cylinder speeds were carried out on the test bench of tangential-horizontal-horizontal threshing and separating device. The preferred operating parameters were obtained by using fuzzy comprehensive evaluation method and variance analysis. The experiments showed that the preferred operating parameters within the test range were feed rate of 5kg/s, speeds of 550r/min, 600r/min, 750r/min and concave clearances of 40mm, 35mm, 40mm for the first tangential cylinder, the second horizontal cylinder and the third horizontal cylinder. The performance indicators were obtained under the above parameters: un-threshed rate of 0.05%, entrainment loss rate of 0.36%, the total loss rate of threshing 0.41%, power consumption of the first tangential cylinder, the second horizontal cylinder and the third horizontal cylinder were 3.33kW, 21.26kW and 12.58kW. The total power consumption was 37.17kW and the extractions impurity rate was 14.37%.