An air-suction metering device for broad beans with a flat belt auxiliary seed-filling device was designed to address the difficulties of sowing large-sized and highly different three-axis dimensional broad bean seeds. The motion mechanism of the flat belt auxiliary seed-filling device and seeds was elucidated through a dynamic analysis of the seed-filling process. A one-factor test was conducted by using the computational fluid dynamics and discrete element method bidirectional coupling simulation method (CFD-DEM) to determine the main component parameters affecting the seed-filling performance and clarify the mechanism of the flat belt auxiliary seed-filling device. An experimental platform was constructed, and a quadratic regression orthogonal combination test was conducted with operating speed, flat belt input shaft speed, and negative pressure as the test factors and qualified index, reseeding index, and miss-seeding index as the test indicators. The test results showed that the primary and secondary factors affecting the qualified index of the air-suction metering device were operating speed, negative pressure, and flat belt input shaft speed. A multi-objective optimization of the test results yielded the optimal parameter combination of the operating speed at 5.69km/h, flat belt input shaft speed at 395r/min, and negative pressure at 3845Pa. The air-suction metering device performance was verified through sowing tests with a qualified index of 91.6%, reseeding index of 3.8%, and miss-seeding index of 4.6%, which met the requirements for broad bean planting.