Abstract: Seed feeding device determines the seeding performance of the air?assisted centralized metering system.Seed filling performance is the key parameter to affect feeding quality of seed feeding device.As seeds′ flow properties and compressive force significantly affect seed filling performance, the effects of seed layer thickness on filling performance for rapeseed and wheat were studied in this research.EDEM(engineering discrete element method) software was used for simulating the filling performance and high speed camera technology was used for executing the experiment.Adjustable regulating plate was adjusted for seed layer thickness under different treatments.In EDEM simulation, the experimental factors were inclined angle of adjustable regulating plate with 3 levels(ranging from 50° to 70°), longitudinal distance of regulating plate with 6 levels(ranging from 0 to 25 mm), lateral distance with 3 levels(ranging from 42 to 50 mm) and rotational speed with 5 levels(ranging from 10 to 50 r/min) in single?factor test.Seeds′ compressive force, tangential force and number of seeds in model?hole were used as test indices in simulation.Bench test was performed on test rig by using Huayouza 62 and Zhengmai 9023 as materials, while field experiment was carried out in the modern agriculture science park of Huazhong Agricultural University in 2015.The 3 inclined angles of adjustable regulating plate, 2 longitudinal distances and 3 lateral distances were evaluated for seed filling performance at 5 rotational speeds (10, 20, 30, 40 and 50 r/min).Angles of adjustable regulating plate were 50°, 60° and 70° using complete randomized design (one?factor), longitudinal distances were 15 and 20 mm and lateral distances were 42, 46 and 50 mm with two?factor full factorial test.Seeds′ initial filling angle, filling angle which was measured by high speed camera using pco.dimax HD system, seed feeding rate, coefficient of variation of seed feeding rate and damage rate were dependent variables in bench experiments.The relationships between seed feeding rate, coefficient of variation of seed feeding rate and seeds′ compressive force, tangential force, filling angle were analyzed.Results showed that: 1) Seeds′ compressive force, seed filling angle and feeding performance of regulating plate with the angle of 60° were better than other parameters.Seeds′ compressive and tangential force increased with the increase of longitudinal distance and the decrease of lateral distance.Coefficient of variance of seeds′ number decreased and then rose up.Seeds′ compressive force was generally stable and tangential force increased with the increase of rotational speed.The number of seeds in each model?hole decreased by 5% with the rotational speed increasing of 10 r/min.2) When rotational speed was within the range of 10~50 r/min, seed initial filling angle, filling angle and seed feeding rate rose with the increasing of longitudinal distance and the decreasing of lateral distance.In addition, the coefficient of variation of seed feeding rate decreased and then rose up with the increasing of longitudinal distance and the decreasing of lateral distance.Seeds′ compressive force, tangential force and seed filling angle were positively and significantly correlated with seed feeding rate.Effects of seed layer thickness and rotational speed on seed filling angle mainly resulted from compressive force and tangential force, respectively.Regarding the agronomic requirements, the ideal parameters were longitudinal distance of 15 and 20 mm for rapeseed and wheat, and lateral distance of 46 mm.Under the combination condition of optimum seed layer thickness, the coefficient of variation of seed feeding rate and the damage rate for the seed feeding device were less than 1.0% and 0.1%, respectively.3)Field experiments demonstrated that planting density was 38~50 plants per square meter and their coefficient of variation was 10.64% using the air?assisted centralized metering device under the condition of optimum seed layer thickness.Results showed that the use of this metering device made the emergence performance of rapeseed meet the agronomic requirements.This study investigated the mechanism of seed layer thickness which affected seed filling performance in seed feeding device for rapeseed and wheat.The investigation of seed layer thickness in this research can provide the basis for optimizing the structure of regulating plate and improving the feeding performance.