Based on hourly observation data from 2005 to 2017 and three-hour observation data from 1969 to 2017 in Lhasa station, diurnal variation characteristics of precipitation during flood season (from May to September) and the long-term evolution characteristics of daytime and nighttime precipitation were revealed. The results are as follows: (1) The diurnal variations of hourly precipitation amount and frequency in Lhasa were characterized by a single peak distribution, both peak values occurred at 05:00 and the valleys occurred during 15:00-17:00, while the hourly precipitation intensity presented a double peak distribution, the peak value occurred at 17:00 and 00:00, and the valley value occurred during 13:00-15:00. (2) The diurnal variation phases of hourly precipitation amount and frequency of different grades precipitation were different. The diurnal variation of hourly precipitation amount and frequency of slight rain and light rain presented single peak  distribution, the peak value occurred at 05:00, and the time of peak hourly precipitation amount and frequency of medium rain and above were earlier than that of the former two grades precipitation. (3) The daytime and nighttime precipitation amount and intensity increased significantly, while precipitation days had no obvious trend in Lhasa during 1969-2017. The main reason of precipitation amount increase was precipitation intensity increase in Lhasa during the flood season.