The atomization of the droplets generated by a flat fan nozzle has been studied in the IEA-I high speed wind tunnel at NERCIEA with Marvern Spraytec Laser Diffraction system. The measurement point is set at 0.15m, 0.25m and 0.35m away from the orifice of the nozzle. The wind speed range is from 150km/h to 305km/h, and the tube pressure is set about 0.3MPa, 0.4MPa and 0.5MPa. The measuring distance from the orifice of the nozzle is found important to the diameter and relative span of the droplets. In our results, 0.35m away from the orifice of the nozzle is found to be a proper measuring point, where the droplets are fully atomized and the flow quality of the wind tunnel is good enough for the test. The effect of airspeed and pressure has been analyzed with response surface method, and the results showed that when the wind speed increase from 120km/h to 305km/h, the Dv0.5 of the droplets decreased from 220μm to 130μm. The increasement of the tube pressure will strongly decrease the droplet diameter at a lower wind speed and slightly increase the droplet diameter at a higher wind speed.