Using daily precipitation and air temperature data of Lhasa to analyze the nightrain rate in rainy season (May-September) during years 1955-2007, the results could be summarized as follows: The variations of nightrain rate was comparatively stable in whole rainy season, without obvious interdecadal changes during the past 50 years. The nightrain rate was significantly associated with daily precipitation, and the nightrain rate increased with the enhancement of precipitation when rainfall was below 25 mm, it was the minimum 75.2% when daily rainfall was below 1 mm, and it was the maximum 93.4% when daily rainfall was 25 mm. There is a significantly negative correlation between night rain rate and daily temperature. In the meantime, the nightrain is related with the terrain. The high nightrain rate has its advantageous aspect, but also it will bring some negative influences.