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In order to study seed movement law of seed conveying tube of airassisted centralized metering device for rapeseed and wheat, effects of seed conveying tube’s diameter and length, ration of lateral length to longitudinal length (k), and joint’s type on seed motion characteristics and airflow field were analyzed with EDEM-CFD coupling approach. Impact of seed conveying tube’s structure on seeding performance was studied by bench experiments. The results showed that seed movement direction which was the same as airflow direction in seed conveying tube was in axis direction and seed motion source was the drag force. The seed velocity in seed conveying tube’s outlet was increased first and then decreased with the increase of ratio of lateral length to longitudinal length (k). The each row’s averaged seeding quantity and apiece row consistency variation coefficient of seeding quantity were affected significantly by seed conveying tube’s structure. When seed conveying tube’s diameter, length and k was 42mm, 1.0m and 2/3, respectively, seed velocity, relative velocity between particle and airflow and seed distribution were good with lower pressure loss and better seeding performance. Seed velocity of conveying tube with bends was larger than that of conveying tube with mansard joint. The relative velocity of conveying tube with bends was lower than that of conveying tube with mansard joint. Airflow field and seed distribution was uniform with larger seed velocity and lower pressure loss in conveying tube with bends of 100mm of radius. When rotational speed of seed feed device was within the range of 10~40r/min, apiece row consistency variation coefficient of seeding quantity of rapeseed and wheat was less than 4.0% and 5.0%, respectively. The stability of full seeding quantity and seed damage rate were no more than 1.0% and 0.1%, respectively, for both rapeseed and wheat. The results provided the basis for structure optimization of air-assisted centralized metering system and improvement of seeding performance.