Abstract: The spatial-temporal characteristics of precipitable water vapor and water-vapor flux as well as precipitation conversion efficiency in the Three River Headwaters Region were discussed through correlation analysis,trend analysis,accumulative anomaly and inverse distance weight based on relevant ERA-Interim reanalysis data from 1979—2016. Some conclusions could be drawn: ① The annual average water-vapor fluxes along the longitudinal and altitude directions in the past 38 years were 2.0 kg/(m·s) and 10.3 kg/(m·s),respectively. The altitude growth rate of water-vapor flux was higher than that of the longitudinal one. The altitude change of water vapor was mainly input,whereas the longitudinal change was mainly output. ② The precipitable water vapor increased slightly. The annual and seasonal means were 1 791.6—2 278.9 mm and 122.2—1 134.2 mm,respectively. The spatial difference in different seasons was significantly large. ③ The annual average precipitation conversion efficiency in the study area was 24.6% and it reached the peak (32.8%) in 1989. Constant spatial distribution was observed to seasonal and annual average precipitation conversion efficiencies. Both seasonal and annual average precipitation conversion efficiencies decreased gradually from southeast to northwest. Moreover,a great seasonal difference of precipitation conversion efficiency was recognized. ④ Although there were abundant atmospheric water resources in the study area,the natural precipitation conversion efficiency was low,which indicated the promising potentials for development.