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To solve the problems of low removal rate and incomplete separation for the current longitudinal-axial-flow combined harvester in the case of harvesting rice and wheat, a segmented longitudinal-axial-flow threshing device was designed in this paper. The threshing device mainly comprised a tapered threshing roller, a concave sieve with adjustable threshing strength, a 360° separation device and an electric control system for operating parameters. Through the single factor tests, the optimal switching states of the concave sieve with adjustable threshing strength for rice and wheat threshing was obtained respectively. In order to find out the influence rule of the working parameters of the device on the threshing effects and the optimal parameter combination, the multi-objective optimization tests were carried out. The rotational speed of roller, the angle of deflector, the threshing gap of concave sieve, the separation gap of concave sieve and the feeding rate were selected as the influence factors and the crushing rate, impurity rate and loss rate were selected as the test indexes. The test results showed that the significant sequence of influence of threshing parameters on the crushing rate was the rotational speed of roller, the threshing gap of concave sieve, the angle of deflector, the feeding rate, the separation gap of concave sieve. The significant sequence of influence of threshing parameters on impurity rate was the rotational speed of roller, the angle of deflector, the threshing gap of concave sieve, the feeding rate, the separation gap of concave sieve. The significant sequence of influence of threshing parameters on loss rate was the rotational speed of roller, the angle of deflector, the threshing gap of concave sieve, the feeding rate, the separation gap of concave sieve. Through multi-objective parameter optimization analysis, the optimal working parameters combination for wheat threshing were determined that the rotational speed of roller was 905 r/min, the angle of deflector was 69°, the threshing gap of concave sieve was 18 mm, the separation gap of concave sieve was 19 mm and the feeding rate was 4 kg/s. The field verification tests were carried out using the optimal working parameters combination. The results indicated that the crushing rate was reduced from 1.46% to 1.00%, the impurity rate was reduced from 1.85% to 1.43% and the loss rate was reduced from 1.72% to 1.20% compared with the conventional longitudinal-axial-flow threshing device. All the relative errors between the measured values and the optimized values were less than 5%, the working indicators met the requirements of relevant national standards. The device effectively solved the problems of high crushing rate, unclean threshing and incomplete separation, the research results can provide reference for the structural improvement and operation parameters optimization of the threshing device of longitudinal-axial-flow combined harvester. [ABSTRACT FROM AUTHOR]