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A test stand about the pneumatic conveying system was conducted to explore the mechanism of the effect of seed tube length on the seeding performance of the pneumatic conveying system. On basis of analysis of law of average flow velocity in tubes with different lengths, computational fluid dynamics (CFD) simulation was conducted to get the map of speed flow field in tubes with different lengths. The quadratic regression general rotation combination design was used for the experiment, and the seed application rate and frequency of fans were taken as influential factors. Moreover, indexes of the test included variation coefficient of each row displacement consistency, variation coefficient of total displacement stability and the average velocity of airflow in the seed tube. A total of 24 seed transport tubes were used, with a gradient of 0.25m and a length ranges from 0.50m to 6.25m. The analysis and test results showed that when seed tube length was increased, the gas average flow velocity was decreased, the seed movement speed was decreased, and the corresponding seed quantity discharge was decreased. Therefore, seed tube length had a significant influence on the seeding quantity. When seed tube length was 2.50~6.25m, both the decrease of average flow velocity and the decline of the seed velocity were slowed down, and the seeding amount was changed smoothly. In addition, the variation coefficient of each row displacement consistency was 2.82%~3.88%, and the variation coefficient of total displacement stability was 0.39%~1.28%. Therefore, the seeding evenness was good and could meet the requirements of relevant standards.