Abstract: Abstract: In order to study the relationship of fan rotational speed and waft airflow velocity field distribution of Medicago hispida harvester, the waft airflow channel model is established, which is composed of draught fan, duct and waft airflow area by the UG NX8 software; all the parts of the waft airflow channel model are meshed respectively by using the integrated computer engineering and manufacturing code (ICEM), the grids of volute and impeller passage are unstructured, and the entrance of body, the duct and the waft flow area are meshed by hexagonal structured grids; then all the parts are assembled together. Input the model into Fluent, choose the multiple reference frame (MRF) as the coordinate system, connect the whole basin with 3 pairs of interfaces, take the standard turbulence model to simulate and calculate, and get the results of waft airflow velocity field by the way of chart. The results show that: when the speed of draught fan is 4 000 r/min, the blast capacity of the blow system is 0.1265 kg/s, and it is larger than the required capacity (0.1167 kg/s) of Medicago hispida; when the speed of draught fan is 4 000 r/min, the airflow of most of the ducts converges at the distance of 0.2 m from the air outlet, and the speed of waft airflow is 12-14 m/s which is larger than the required speed (11 m/s) of Medicago hispida, while the distance from the air outlet is 0.3 m, the airflow of the ducts converge entirely, but the speed reduces to about 10 m/s which is below the require, so the waft distance is determined to be 0.22 m by analyzing the area between 0.2 and 0.3 m. In order to identify the simulation result of waft airflow velocity field with computational fluid dynamics (CFD), the experiment of determining waft airflow velocity field is carried out under the fan rotational speed of 4 000 r/min. The environment of experiment is indoor without wind so as to reduce interference. The handheld anemometer Kestrel 4000 is used to measure wild speed, whose measurement range and accuracy are 0.4-40 m/s and ±0.1 m/s respectively, and the non-contact tachometer is used to measure fan's rotational speed, whose accuracy is 1 r/min and measured distance is 50-300 mm. The measure point is signed in the flow area before the test. The test results are compared with the simulation results after the test: the wild speed value from the test is 19.86-23.33 m/s at air outlet, 4.8%-9.3% less than the simulation value, 18.94-20.97 m/s at the distance of 0.1 m from air outlet, 2.1%-9.2% less than the simulation value, 11.94-14.13 m/s at the distance of 0.2 m from air outlet, 2.5%-7.4% less than the simulation value, 7.53-9.35 m/s at the distance of 0.3 m from air outlet, 6.6%-10% less than the simulation value, and 5.6-7.34 m/s at the distance of 0.4 m from air outlet, 6.6%-9% less than the simulation value. The comparison results show that the change trend of experiment value is consistent with simulation value, though slightly smaller than the simulation one. It means that the numerical calculation model established in the research is correct and reasonable, and it can reflect the distribution of waft airflow velocity field truly, so the selection of fan and duct can meet the requirement of blowing of Medicago hispida.