High cirrus clouds have large influence on weather and climate, and its lidar ratio is an important optical property. Suppose that there is only molecules without aerosol above and below the clouds, the transmittance and optical thickness of clouds can be retrieved from lidar data, and the lidar ratio of clouds. Analysis was performed on the clouds data acquired from 532nm lidar of Hampton University. Observation was performed from January to October 2007. 798 samples with optical thickness between 0.08 and 1.5 were selected. Results show that cirrus clouds observed are mostly in 7~13.5 km altitudes. The lidar ratio is 21.4sr in average, the change of the lidar ratio with season is not obvious, the probability distribution function of lidar ratio appears normal distribution feature. The probability distribution function of optical thickness indicates that most clouds are thin clouds.