In order to understand the spatial and temporal variations of cirrus over semi-arid areas in Northwest of China, macrophysical and optical characteristics of cirrus clouds were observed by micro pulse lidar (MPL-4B) over Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL, 35
57
N, 104
8
E). The structures and optical properties of the cirrus clouds as well as their spatial and temporal variations were discussed and analyzed. The results show that cirrus clouds change from thin to thick, observed ranging from 7 to 10 km, with a mean thickness of (2.0±0.5) km. During this period, the samples have temperature between -51℃ and -39℃. The cloud optical depth increases and then decreases with increasing geometrical depth, ranging from 0.084 to 1.649, with a mean value of 0.651±0.403. Lidar ratio of cirrus clouds is (17±17) sr and it’s found that lidar ratio of optically thin cirrus is larger than that of thick cirrus. Thin cirrus clouds with ambient temperature below -45℃ occurr above 8.6 km and its thickness is lower than 1.8 km. The lidar ratio of thin cirrus is between 5 and 69 sr.