Abstract: To provide foundation for tackling climate change and protecting ecology and environment, the humidity change and distribution characteristics of the terrestrial surface during plant growth period on Loess Plateau were deeply analyzed. Based on climatic factors observed on Loess plateau and by using Penman-Monteith Mode, the humidity index of plant growth period was computed and terrestrial surface humidity characteristic was studied. Humidity index decreased successively from southeast to northwest during the of plant growth period in the research region. The abnormal spatial distribution of the terrestrial surface humidity condition during plant growth period was divided into three key regions: northeast part of the plateau, south part of the plateau and west part of the plateau. Apart from northeast part of the plateau, the other two regions have a remarkable descending trend on annual mean terrestrial surface humidity index; each of them had a notable mutation from wet to dry in 1978 and 1979 respectively. In the three spatial distributed regions, the terrestrial surface’s annual humidity index had a remarkable 3-4 years periodic oscillation; remarkable 5-6 years and 7-8 years periodic oscillations also appeared in south plateau and west regions. The desiccation tendency appeared to decrease from the center to the edge of Loess Plateau. The desiccation obviously showed the distribution characteristics extending from the center of the plateau to all around the region.