By using the radiosonde data and precipitation data in Lhasa station from 1969 to 2008, the evolution characteristics of precipitable water and precipitation in summer are discussed.The statistic results indicate that there is a significantly positive correlation between precipitable water and precipitation in summer.The precipitable water and precipitation have the same inter-annual variation and inter-decadal variation, having the periodic oscillation of 3-years and quasi-11-years respectively.In addition, both precipitable water and precipitation show increasing trend in the last 40 years.But the increasing of the precipitation is more obvious and the growth of precipitation is greater than that of precipitable water.The analysis of precipitation conversion efficiency showed that the mean summer precipitation conversion efficiency is 26.06% with evident inter-annual difference.The maximum conversion efficiency value is about three times of the minimum value.In the years with high precipitation conversion efficiency, both the convergence in the low levels and divergence in the high levels are strong, resulting in the strong vertical velocity in Lhasa, thus favors the precipitation formation, and vice versa.