Abstract: Abstract: Aiming to explore the influence of pneumatic roller type seeder on the absorbability of small seeds with varied sizes, the paper performs a theoretic analysis on the relation between the seed that is in critical motion in the airflow field outside the suction pore and the corresponding position of the suction pore. The state of gas-flow in the airflow field outside the suction pore is analyzed as well as the connection between the seed's flow resistance in the airflow field and the seed's distance to the center of the airflow field, and furthermore, a force analysis is conducted on the seeds in critical motion in the airflow field. Subsequently, a mathematical model of seeds' suction-boundary is established. Simple comparison approach is adopted. The experiment conditions are that the diameter of the seed is 2 mm, the vacuum pressure of seed metering device is 4 kPa, the diameter of suction pore is 0.25 mm, and the frictional coefficient of seeds is 0.3; then the parameters above are changed respectively: the diameter of the seed is 1.4 mm, the vacuum pressure is 4 kPa, the diameter of suction pore is 0.4 mm, and the frictional coefficient is 0.01; thus the values of suction-boundary are able to be achieved accordingly. According to a set of suction-boundary values, the suction-boundary as well as its motion region will constantly will remain steady as the diameter of seeds decreases. When the vacuum pressure and the diameter of suction pore increase, the suction-boundary along with its motion region will expand accordingly. When the frictional coefficient is reduced, the suction-boundary along with its motion region will expand as well. In the research regarding the influence exerted by the size of seeds' suction-boundary over the absorbability of seed metering device, the seeds of flowering Chinese cabbage and broccoli along with hot pepper are used as experimental materials with the vacuum pressure of seed metering device ranging from 1 to 12 kPa. Other parameters of the experiment are defined as follows: the diameter of suction pore is 0.25 mm, the vibration amplitude of seed-box is 0.4 mm or without vibration, and the speed of roller is 300 trays/h. The experiment manifests that seeds' continuous entry into the suction-boundary serves as a prerequisite for the suction and absorption, and the absorbability of seeds is related to the size of seeds' suction-boundary as well as the shape and the size of seeds. Conclusions are drawn as follows: (1) If seed-box doesn't vibrate, seeds will arch around the suction pore, and the seed metering device will miss the seeds consecutively. Therefore, different approaches need to be tailored for different seeds. (2) For the vegetable seeds with small size, the rate of single seed for flat seeds is much less than that for round-shaped seeds. The shorter the diameter of round-shaped seed is, the harder the high rate of single seed will be achieved. When it comes to small-sized flat seeds and round-shaped seeds with a relatively short diameter, the appropriate method for seed supply and the increase of vacuum degree for seed sowing device will prevent the suction pore from missing seeds; extra seeds on the suction pore are cleaned up subsequently. The functions can make the approach effectively achieve precision seeding of the seed metering device.