Aiming at the problem of lacking dedicated harvesting equipment and having low clean harvest rates for small-seeded alfalfa in mechanized harvesting both domestically and internationally, a half-feed alfalfa seed harvester combining mechanical combing separation with negative pressure airflow suction and flow enhancement was designed. Based on the biological characteristics of alfalfa plants and planting agronomy, a mechanized harvesting scheme was proposed, including mechanical combing separation, airflow adsorption and transportation, and enhanced airflow through a fan for small-seeded alfalfa. According to the mechanism of loss suppression during the alfalfa seed harvesting process, key components of the combing platform were designed. For uncombed losses during the alfalfa seed harvesting process, a combing structure was designed to perform combing separation of highly dispersed seed pods. For seed fall and splash losses, a drum-wall combination airflow throat and negative pressure fan system were designed to adsorb and transport the seeds and pods. On this basis, a kinematic analysis of the combing detachment process was conducted. It was determined that the effective range for the comb detaching speed ratio was between 36 and 50, with a comb detaching drum radius of 290 mm. The critical parameters affecting incomplete combing losses were identified as the rotational speed of the comb detaching drum, the center-to-ground height, and the forward speed of the harvesting platform. Using CFD simulation combined with CCD orthogonal experimental methods, the interaction effects of the combing drum rotational speed, airflow throat dimensions, and negative pressure fan speed on the platform airflow field were explored. Using response surface methodology, the optimized values for the airflow throat dimensions, combing drum rotational speed, and negative pressure fan speed were determined to be 83 mm, 1 000 r/ min and 1 450 r/ min, respectively. To verify the effectiveness of the whole machine and the combing platform function, field trials of the alfalfa seed harvesting prototype were conducted. Results showed that when the height of the combing drum center above ground level was 430 mm and the machine’s travel speed was 0.68 m / s, there was virtually no uncombed seed pod after combing, the alfalfa seed clean harvest rate reached 83.4% , and the operational efficiency was 0.73 hm 2 / h, meeting the mechanized harvesting index requirements for alfalfa seeds, providing a reference for the design of small-seeded seed harvesters.