Abstract: Accurately separating a single seed from the seed population by vacuum suction precision is the core advantage for a precision pneumatic seed metering device. For spherical small particle size seeds such as rapeseed, vegetable seed, investigating on the influence mechanism of vacuum singulation process of pneumatic seed metering device through the qualified index and missing index is susceptible to subsequent seed discharging and seed guiding processes. The purpose of this research was to explore the vacuum singulation process of a precision pneumatic seed metering device which was innovated specifically for spherical small particle size seeds by employing negative vacuum and positive pressure to singulate and discharge the seed respectively. With different parameter combinations of grouped seeds, hole diameter, seeding disc rotate speed and negative pressure, the photos of seed suction and migration state on seed metering device were photographed. It were statistics and analysis that the probabilities of the holes no seed holding, single seed holding and multiple seeds holding in the seeding disc. 5 sets of seeds with different thousand seeds weight were obtained by seed screening pretreatment. The analysis of seed physical properties showed that there were large relative differences although these small-sized seeds had small particle size and light weight. The experiment revealed that small particle size seeds could be held by the hole at -200 Pa because they were light in weight, 4-6 multi-seeds holding would occur when the absolute value of negative pressure was too large. The analysis showed that the probability of single-seed holding was a highly significant correlation with factors such as thousand seeds weight, the hole diameter of seeding disc, the rotate speed of the seeding disc and negative pressure. There was an applicable negative pressure range as 1-2 times of the critical negative pressure to ensure the seed metering device to separate a single seed accurately, by which the single seed holding probability of the device was higher than 0.92, and the probabilities of no seed holding and multiple seeds holding were less than 0.04. Force analysis of the suction process showed that the critical negative pressure was related to mechanical and physical properties of seeds, hole diameter, rotate speed of seeding disc and suction reliability. The main mechanism of the variation of seeding disk rotate speed affecting the seed holding was that the fast rotation speed reduced the effective time of the hole and the seed, thereby degraded the single seed holding reliability. When the rotate speed was faster, the absolute value of the critical negative pressure to accurately separate a single seed should increase. Therefore, the reliability coefficient to calculate the negative pressure should be increased. In order to quantify the reliability coefficient of seed holding by vacuum, a nonlinear fitting calculation method was applied. The 1st, 3rd, and 5th set seeds were divided into the correction set for the fitting calculation and the 2nd and 4th set seeds were divided into the verification set. The working negative pressure with no seed holding probability 0.02 and single seed holding probability 0.92 were selected as the reference value. A mathematical model of the reliability coefficient was established with rotate speed of seeding disc and hole diameter as dependent variables. The determination coefficient of the calibration set and the verification set of the mode were both greater than 0.82, which proved the reliability coefficient could be determined by using this model. Then the critical value of negative pressure could be worked out, by which the no seed holding probability kept less than 0.04, the single seed holding probability kept more than 0.92 and the seed metering device worked at optimal status. The research clarifies the influence mechanism of positive and negative pressure combined precision metering device for small particle seed accurately separating process and provides a reference for design and performance improvement of pneumatic seed metering device.