In order to study the cloud base features of Tibetan Plateau,the in tensive observation of clouds and precipitations in Naqu area (31.48°N,92.06°E,4508.9m above mean sea level) was conducted from July 7to August 30,4with the support of the Chinese Third Ti betan Plateau Experiment of Atmospheric Sciences.A laser ceilometer CL31by Vaisala was used to observe cloud base heights and layers continuously for a total time of 55days.Prelimin ary statistical analysis results show that the data missing rate during all measured days is 0. 4%,and almost 24h full coverage observation is achieved.The maximum observation range of the ceilo meter CL31is 7.6km.All the observed data from 0to 7.6km above ground level are averaged pe r hour.The final statistical results show that the cloud appearance probability is 78% duri ng summer in Naqu, and the rest situation is cloudless case.The cloud appearance probabilities of s ingle-layer,double-layer and triple-layer clouds are 65%,12% and 1%,respectively.The single-layer clo ud is dominant and it has obvious diurnal variations.Clouds mainly distribute be tween 2km and 6km from ground height,and the average cloud base height is 2.582km.This conclusion doe sn′t include the clouds above 7.6km which can′t be detected by the CL31ceilo meter.