The complicated cotton picking process was analyzed and numerical modeling for the pneumatic conveying system was carried on. The slip grid and UDF in the Fluent software were used to control the movement of each part of the cotton picking device and to realize the flow field analysis during the movement. The internal flow field change was simulated through the dynamic change of the grid. The results indicate that when moving to 25 s at different outlet speeds, the inlet speed of the suction cotton conveying device has a greater influence on the flow field inside the cotton picking device, it is necessary to reasonably control the speed of the suction cotton conveying device to convey the seed cotton; while the outlet speed of the seed cotton is at least 25 m/s under the conditions of pneumatic conveying, the speed and pressure cloud diagram inside the cotton picking device are more evenly distributed; the speed is higher when the airflow direction in the cotton conveying area is the same as the direction of the docking brush. The numerical modeling of the cotton picking device and the analysis method of the flow field can provide reference for the subsequent design and optimization.