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A pneumatic seeding downforce regulating device was designed to reduce pressure fluctuations caused by surface undulations during mechanized sowing operations, which reduced the stability of seeding depth. The motion process of downforce regulating device was analyzed, and the torsional deformation process of the air spring which was the main working component was clarified. The main structural parameters of the air spring that affected the seeding downforce stability were determined through analyzing the influencing factors of seeding downforce and the deformation process of the air spring, including cord angle, piston radius, and piston angle. In order to determine the optimal parameter combination, a finite element simulation model for gas-solid coupling of air springs was established. Taking improving the downforce stability as the optimization index, a quadratic rotation orthogonal combination simulation experiment was conducted, and a regression model of test indicators and influencing factors was established. Following the principle of reducing the vertical stiffness and ensuring that the vertical output downforce of the air spring met the requirements, the optimal parameter combination for the downforce air spring were determined by simulation experiment: cord angle was 38°, piston radius was 42mm, piston angle was 23°. To verify the effectiveness of theoretical analysis and simulation experiments, the field experiments were conducted on the pneumatic seeding downforce regulating device under the optimal parameter combination, the experimental results showed that the pneumatic seeding downforce regulating device can effectively improve the stability of ditch depth compared with the spiral spring seeding downforce regulating device. When the operating speedwas 4km/h, 8km/h, and 12km/h, the qualified rates of ditch depth were increased by 8, 3 and 11 percentage points respectively, reducing the coefficient variation of ditch depth by an average of 2.58 percentage points, which significantly improved the consistency of seeding depth during mechanized seeding.