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Seeding absence caused by negative pressure drops is a major technical problem in pneumatic precision metering device. A novel design of a soybean precision metering device was presented, which was compatible with pneumatic and mechanical combination, that consisted primarily of a plate, a covering, a shall and a shaft. The plate of the pneumatic-mechanical combined seed metering device had three seed picking institutions, suction hole, guide groove and pickup hole. To ensure seeding performance in the case of negative pressure drops, three institutions worked together to change the movement of soybean seed. The discrete element simulation software EDEM was used to simulate to optimize the geometric structure parameters of the guide groove, and the Hertz-Mindlin no-slip contact model was selected for the simulation. Seed speed, seed number and miss filling rate were used as test indexes for the simulation. The optimal angle and depth of the guide groove were 45° and 2mm, respectively. The soybean seeds of Jiyu 302, which was widely adopted in Jilin Province, were used as an object of indoor bench test and field trial. The indoor bench test was carried out on a JPS-12 test bench, which was widely used to research the performance of seed metering device. Seed hole size, forward speed and negative pressure were chosen as test factors. Qualified rate, multiple rate and missing rate were used as test indexes. The variance analysis, as well as regression analysis of the test data, was carried out by Matlab software. The results showed that seed hole size, forward speed and negative pressure significantly affected the seeding performance. The optimal geometric parameters of the seed metering device were as follows: the up edge width of the seed hole was 9.5mm, the width between front and back face was 9.5mm, the below edge width and depth was 7.3mm and 5.7mm, respectively. With those optimal geometric parameters, the qualified rate of the pneumatic-mechanical combined seed metering device was no less than 95% when the forward speed was no greater than 8.6km/h and the negative pressure was no less than 1.6kPa. Field trials were carried out in the experimental field of Agricultural Machinery Research Institute in Changchun, Jilin Province. A 2BGD-6 pneumatic precision planter was used in the field trials, which was designed by Jilin University and manufactured by Heilongjiang Bonong Machinery Co., Ltd.. The following test results were obtained: the qualified rate was 93.67%, the multiple rate was 3.32% and the missing rate was 3.01%. Furthermore, another bench test was carried out to compare among the pneumatic-mechanical combined precision metering device, a traditional air-suction type metering device and a MASCHIO air-sucking type metering device. The comparative bench test results showed two points: firstly, when the negative pressure was dropped to 1.1kPa, the qualified rates were improved by 6.48 and 1.92 percentage points, respectively, compared with traditional and MASCHIO metering device. On the other hand, when the negative pressure was dropped to 0.6kPa, the qualified rates were improved by 9.12 and 4.25 percentage points, respectively.