Based on the time series of temperature and dew point depression from the four radiosonde stations in the Heihe River basin during 1981-2010, the climate characteristics of specific humidity along vertical profile and the annual and decadal changes of seasonal and annual specific humidity at each standard isobaric surface were studied, and the seasonal contribution to the decadal changes were analyzed. The results showed that the water vapor mainly concentrated in the lower troposphere, the maximum specific humidity appears in summer, with the average specific humidity accounting for only 0.4 to 0.77 of the average in China, and the specific humidity over 200 hPa was approximately to the average in China, with less seasonal change. Except that the summer specific humidity at the 700 hPa was rising, the annual and seasonal ones were on the decline, and its relative decreasing rate along the profile was increasing gradually. The annual and seasonal specific humidity at typical isobaric 700 hPa, 500 hPa and 200 hPa had risen first and then dropped, experiencing the "dry-wet-dry" process, taking the middle of the 1980s and 2002 as boundaries. The maximum over the period from 1981 to 2010 appeared in 2002 and the change amplitude decreased with height. The winter humidity anomaly changed very stably and specific humidity in summer fluctuates very intensely. The specific humidity in the study area all increased in the 1990s, but decreased in the 2000s. In the 2000s, there was a sharp decline of specific humidity mainly resulted from the contribution in summer but statistically the most significant contribution in autumn and it was 10 times as much as the increase in 1980s mainly contributed in autumn and 5 times as much as 1990s, mainly in summer and autumn.