Abstract: Abstract: The tractor load keeps varying randomly during the tractor - pneumatic planter unit working. It could cause the air pressure in pneumatic precision seed-metering system to become not as constant as the design due to the working speed fluctuation of air pump on the planter led by the variation in rotation velocity of the tractor's power output shaft. Consequently, it would deteriorate the distribution uniformity of seeds in the field and worsen the growth and final yield of crops. To solve this problem, an effective air pressure stabilizing method of pneumatic seed-metering system was proposed based on spring-loaded air pressure relief valve. The first step was to measure the rotation velocity of the tractor's power output shaft in the field. After that, the relationships of the speed of the air pump and the air pressure of the pipeline and the seed-metering device were determined through the bench test. Then, the predetermined control point value of air pressure relief valve was estimated, and the structure and working parameters of air pressure relief valve spring were calculated and selected. Additionally, theoretical analysis and experiments were performed to demonstrate the efficiency of the selected relief valve. A practical engineering in precision combined rapeseed planter, called 2BFQ-6, was used to demonstrate the application of proposed air pressure stabilizing method. A field test was conducted to determine the working speed of the air pump and a laboratory test was implemented to confirm the relationships of the air pump working speed, air pressure in the pipes and air pressure in the chamber of seed-metering device. The test results showed that the coefficient of variation of working speed of the air pump was less than 4.38% when the tractor was working with the same gear, and the coefficient of variation was less than 9.2% as the tractor was working with different gear. To ensure persistent seeding of the seed-metering device, the working speed of the air pump should be in the range of 2 020-2 620 r/min. In order to stabilize air pressure of the pneumatic seed-metering system, the predetermined point value of relief valves were 1 000 and -5 500 Pa with positive and negative air pressure respectively. A type of spring-loaded air pressure relief valve was selected and its construction and working parameters were calculated and analyzed. Carbon steel wire cylindrical spiral spring was employed in the valve. The intermediate diameter and the pitch of the spring were 30 and 10 mm respectively. The number of active coils of the spring was 8. The line diameter of the spring was 1.0 mm for positive pressure relief and 1.5 mm for negative pressure relief. The precompression distance of adjustment bolts were 6.7 and 7.8 mm for positive and negative pressure relief respectively. Theoretical analysis based on air pressure - flow rate characteristics verified that the relief valve used in this pneumatic seed-metering system could be used for air pressure adjusting. Furthermore, air pressure and seeding quantity experiments were performed to evaluate the performance of the selected valve as well as to confirm the feasibility of the proposed stabilizing method. The results showed that the positive and negative air pressure of the seed-metering device could range from 200 to 350 Pa and from -3 900 to -3 000 Pa respectively by employing the relief valves, under which the pneumatic seed-metering system could work stably, showing a contrast to the speed fluctuation of the air pump. The air pressure stabilizing system decreased the deviation rate of the positive and negative air pressure in the air chamber of seed-metering device by 45% and 110%, respectively, which facilitated the pressure-regulating response sensitivity and stability to satisfy the requirement. With the increase of the air pump working speed from 2 000 to 2 700 r/min, the coefficient of variation of seeding quantity was 3.84% without the air pressure stabilizing. However, it was reduced to 1.16% when the air pressure stabilizing method with the customized valves was applied, which decreased by 2.68%, indicating the improvement of seeding quantity stability. The research has confirmed that the air pressure stabilizing method by employing air pressure relief valve can effectively solve the problems that the working pressure in the seed-metering device fluctuates when the rapeseed planter works in the field. The method can provide a technical reference for pneumatic planter design and optimization.