Abstract: An air-suction seed metering device has been widely used for precision planting in various crops, due to the minimal seed damage, high efficiency, simple structure and strong adaptability. Single-seed broadcasting has been one of the most important capabilities to improve the quality of hybrid rice planting, particularly with the direct seeding of hybrid rice and the seed production levels. However, the stable filling and high precision of single seed broadcasting are highly required to meet the large amount of sowing in the air-suction seed metering device in recent years. In this study, an improved air-suction single-seed metering device was proposed with rectangular suction holes and auxiliary seed filling. Taking the "Yoshida" hybrid rice as the research object, the gravity distribution of the seed was analyzed to optimize the structure parameters. It was found that the high adhesion of the seed depended mainly on the rectangular shape of the seed suction hole on the seed-sucking plate. According to the fluid-solid coupling theory in CFD-DEM, Ansys Fluent and Rocky Dem software were used to simulate the airflow part of the planter and the interaction between the planter and seeds. A CFD-DEM simulation model was then established to exchange data for the air-suction single-seed metering device. Five types of single-factor experiments were conducted with the seed suction holes in the same area. The drag force, pressure gradient force, and air-suction force were taken as the experimental indicators. The seed suction hole with the maximum air-suction force was optimized as the size of 0.8 mm × 2.25 mm. In this case of the seed suction hole, the auxiliary filling angle, working speed, and working pressure were selected as the experimental factors, with the single rate (S), multiple rate (M), and leakage rate (L) as the experimental indicators. The optimal ranges of auxiliary filling angle, working speed, and working pressure were determined to be 70°-90°, 30-60 r/min, and 400-800 Pa, respectively. Subsequently, the Box-Behnken experimental design was conducted to combine with the variance analysis, response surface method, and multi-objective optimization. The variance analysis indicated that the primary and secondary influencing factors on the single rate were the auxiliary filling angle, working pressure, and their interaction term. The primary and secondary influencing factors on the multiple rate were the auxiliary filling angle, working pressure, the interaction term between the working speed and working pressure, and the interaction term between the auxiliary filling angle and working pressure. The primary and secondary influencing factors on the leakage rate were the auxiliary filling angle, working pressure, and the interaction term between the auxiliary filling angle and working speed. The response surface analysis showed that the single rate had a strong correlation with the interaction term between the working pressure and auxiliary filling angle. The multiple rate had a strong correlation with the interaction terms between the working pressure and auxiliary filling angle, as well as between the working pressure and working speed. The leakage rate had a strong correlation with the interaction term between the auxiliary filling angle and working speed. The multi-objective optimization showed that the better performance of the seed metering device was achieved in a single rate of 86.91%, a multiple rate of 9.46%, and a leakage rate of 3.63%, when the auxiliary filling angle was 80.90°, the working speed was 42.65 r/min, and the working pressure was 621 Pa. The experimental verification showed high consistency with the optimized, with a single rate of 10.23%, a multiple rate of 9.46%, and a leakage rate of 3.41%. The research findings can provide better guidance to optimize the air-suction single seed metering device, in order to improve the overall operational accuracy for direct rice seeding machines.