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Most tractors in the world still use manual mechanical shifting transmissions. Such transmissions face problems for instance power interruption and cumbersome operation during gear shifts, which is extremely unfavorable for vehicles with load operations such as construction machinery, which not only increases the fatigue intensity of the driver, but also reduces productivity. To ensure the maximum flexibility of use at each speed and exploit the maximum engine power available under all working conditions. A series hydraulic and mechanical hybrid transmission system (HMD) and its supporting tractor overall design method were proposed, including functional requirements, transmission route design, component characteristics analysis, performance parameter calculation, simulation model building, and operation performance comparison. The system scheme was based on the series combination of hydraulic torque converter (TC) and dual clutch transmission (DCT). Based on the proposed design method, the Dongfanghong 1804 tractor was selected as the research object. According to the effectiveness of stepless speed regulation module, the traction performance and matching performance test were designed. Reasonable engine matching made the maximum efficiency of hydraulic transmission reach 0.894, and the stepless variable speed can be realized in heavy and light load sections, and the adaptability of working conditions was excellent. The comparison results showed that the traction characteristic field of HMD was located between the original tractor and HMCVT, and the peak amplitude of tractive force in each gear varied smoothly and the area of deep valley was significantly reduced. Under ploughing tillage, traction coverage was increased by an average of 15.3%, and the load operation speed was increased by an average of 8.2%. Under rotary tillage, the take-off power was increased by 1.7% on average. The research can provide a technical route reference for the overall design of multimodal infinitely variable speed tractors.