Abstract: An air suction seeder has been widely used in the large-scale planting of rice, corn, vegetables, and rape, as well as the indoor factory seedling production, due to the high sowing accuracy, strong adaptability to seed size, low seed injury rate, high operating efficiency, and low cost of use. Among them, the seed metering device and air power system are two important core components of air suction seeders. Therefore, their operation can greatly contribute to the performance of the whole machine. It is also necessary for the stable air pressure supply and regulation in the air suction seed metering device pneumatic system. As such, the seeder can fully adapt to the sowing of various crops and have stable operation. Particularly, the multi-branch convergence pipe is the key component of the seeder pneumatic system. The cooperative operation of one seeder and multiple rows of seeders can be realized to converge the negative pressure tributaries that are generated by multiple seeders into the total flow and then conveyed them to the pneumatic system fan. The internal geometric structure can also be optimized to improve the working performance of the air suction seeder. The mechanical structure of the seeder is no longer the main reason for the increase in pressure loss in the pneumatic system and energy consumption of the fan. However, energy loss can be produced, when the airflow is more likely to mix with each other at the tee position at the junction of the multi-branch pipe header and the branch pipe, as the airflow of the pneumatic seeder is restricted by the geometric mechanism of the multi-branch convergence pipe in the process of spatial transfer. The accuracy and rationality of the piping structure can be the key issue to reduce the pressure loss and energy consumption of the pneumatic system. Therefore, it is essential to explore the pressure loss of airflow in the multi-branch convergence pipe and then to reveal the fluid motion state in the process of manifold piping for the low-energy multi-branch convergence pipe structure. In this study, a systematic investigation was implemented to clarify the flow mechanism of negative pressure airflow in the multi-branch convergence pipe of an air-suction seeder pneumatic system. The correlation characteristics were obtained between the total flow pressure loss and pipe geometry, in order to determine the quantitative prediction target value of total flow pressure loss. The flow state of the multi-branch convergence pipe was also analyzed to clarify the main influencing factors on the flow of the pipe. A single-factor experiment was performed on the Fluent simulation software. The flow mechanism was also established to explain the airflow pressure loss in the multi-branch pipe and the hydrodynamic mechanism from the microscopic perspective. The dimensional analysis was implemented to determine the pressure drop (ΔP, Pa) of outlet branch pipe and air density (ρ, kg/m³), dynamic viscosity (μ, Pa·s), length of the closed end of the header pipe (h, mm), the flow rate of inlet branch pipe (Q, m³/s), the inner diameter of inlet branch pipe (d, mm), length of inlet branch pipe (l, mm), spacing of inlet branch pipes (δ, mm), the inner diameter of header pipe (γ, mm), outlet branch pipe inner diameter (D, mm) and outlet branch pipe length (Δ, mm). The bench test results show that the application range of the established empirical equation formula were 0.0009 m³/s≤Q≤0.0045 m³/s, 28.0 mm≤d≤45.2 mm, and 100 mm≤ l ≤ 200 mm, 200 mm≤δ≤300 mm, 42.6 mm≤γ≤81.4 mm, 150 mm≤Δ≤ 250 mm, 34.0 mm≤D≤42.6 mm, and 53.6 mm≤D≤57.0 mm. The prediction accuracy of the total flow pressure drop can be controlled within 10% of the calculated by the empirical formula. The established empirical formula can provide a strong reference for the design selection and structure optimization of multi-branch convergence pipes of air-suction seeders.