Soil moisture is the link between the land surface and the atmosphere, which plays an important role in the hydrological cycle. As the "Third Pole" and "Asian Water Tower",the Tibetan Plateau has an important influence on the climate of the surrounding areas such as the formation and maintenance of the Asian monsoon and it also profoundly affects the availability of Asian water resources. Based on the measured soil moisture data from 100 stations distributed in the three climate zones on the Tibetan Plateau, this paper assesses the ECV,ERA, MERRA and Noah datasets, selects the best evaluated dataset for surface soil moisture, and analyzes the influence of various meteorological factors on spatial and temporal patterns of soil moisture changes. Finally, the paper evaluates the changes of surface soil moisture during the next about 100 years and explores possible climate causes. The results show that: (1) The Noah dataset has the best assessment of surface soil moisture in the Qinghai-Tibet Plateau during the historical period. Among all the regions, Naqu obtains the best assessment of surface soil moisture in each dataset. (2) Among various meteorological factors, precipitation is the most important factor affecting the temporal and spatial patterns of soil moisture in most areas, but the temperature and solar radiation have a relatively high impact in the Himalayas, especially on the north slope of the mountains. (3) The surface soil moisture had a significant downward trend from 1948 to 1970. However, it did not fluctuate obviously from 1970 to 1990. From 1990 to 2005,there existed a certain upward trend. Conversely, it has a rapid downward trend since 2005. (4) There is a downward trend for surface soil moisture in different future scenarios. Compared with the RCP2.6 and RCP4.5 scenarios, the soil moisture declines obviously with a more significant downward trend after 2080 under the RCP8.5 scenerio. (5) In the future, both precipitation and temperature show an upward trend. There was a downward trend for the drought index in the RCP8.5 scenario, whereas, there is no significant change under the RCP2.6 and RCP4.5 scenarios. The drought index can explain the change of surface soil moisture in the future to a certain extent.