The airborne PMS particle detection system is used to observe a rainfall process occurred in central Gansu Province on 27 August 2006, and the microphysical features of the precipitation clouds are analyzed. The results show that the clouds mainly consist of small cloud particles with the average diameter of less than 20 μm, which have more contribution to cloud water content. It obeys the “sowsupply” mechanism of precipitation, and cloud droplet concentration and water content at lower levels were relatively greater, but the average diameter was less than that at upper levels. There were three growth mechanisms of cloud droplets: deposition growth and riming growth at the top of the cloud, and mainly coagulation growth in the lower part of the cloud.