The orographic clouds structures of Qilian Mountains are analyzed based on the data of cloudage, cloud types, atmosphere vapor, wind field, raindrop spectrum, and rainfall intensity observed during 2006 and 2007 summer field campaign for Qilian Mountains Orographic Clouds Observation Study in the west part of Lenglongling in Qilian Mountains. The results indicate that: i. the cloudage is abundant in summer in Qilian Mountains, more than 60% on average and total cloudage is about 80% under southwest airflow pattern conditions. ii. Atmosphere vapor in Qilian Mountains is very rare during the no-rainfall days in summer, the atmosphere relative humidity above 700 hPa level is mostly less than 20%. iii. The airflows converge toward mountaintop during diurnal time while diverge toward valley during nighttime by due to the co-effects of mountain valley winds from south and north of Qilian Mountains under southwest airflow pattern conditions. It is very easy to uplift to form orographic clouds which is prone to produce precipitation when water vapor abundance. iv. The rainfalls in Qilian Mountains mostly come from the raindrops with diameters less than 1 mm.