In view of lack of special mechanical harvesting equipment, a rotary cutter disc flat stubble cutting device of King grass harvester was designed. According to the biological characteristics of King grass and the harvest requirements of flat stubble cutting, the overall structure design of the cutting device was completed. Through theoretical analysis, the key structures and working parameters of cutter and conveyor were determined. ANSYS/LS-DYNA was used to simulate and compare the cutting effects of single moving knife cutting and moving fixed knife combination cutting. The results showed that the flatness of cutting section with moving and fixed cutter was better than that with single moving cutter. When the cutter head was cutting at low speed, the cutting power consumption of the combination of moving and fixed cutter was less than that of single moving cutter. The conveyor had good conveying effect and can realize the continuous stubble conveying of King grass stalk after cutting. The cutter test-bed was built. Through NSGA-Ⅱ algorithm, the optimal parameter combination of cutter was determined: the blade clearance was 2.98mm, the cutter head speed was 84.7r/min, and the cutter head inclination was 28.65°. Under this parameter combination, the stubble breaking rate was 8.66% and the unit cutting power consumption was 7.78mJ/mm2. On this basis, the field experiment was carried out, and the experimental results were basically consistent with the optimization results. The results showed that the overall harvest quality was better when the rotary cutter disc flat stubble cutting device was applied to the harvest of King grass.