Temporal heterogeneity of resources supply is as important a factor as environmental heterogeneity (spatial) to vegetation dynamics and landscape evolution. Mountain ecosystems in arid and semiarid regions are frequently characterized by significant spatial self-organization, and temporal heterogeneity of soil moisture maybe an important driver factor in the vegetation pattern formation and maintenance. A series of observations and experiments were conducted near the forest boundary in Qilian Mountain to test the differences in daily variability of soil moisture between grassland, shrubland and forest habitats, and to examine the contributions of canopy interception and plant uptake to any observed differences. The results show that: daily measurements of soil moisture during a month of growing season showed strong interactions between habitat and time(F2,27 =11.25，P <0.01 for 5cm depth; F 2,27 =5.51，P<0.01 for 20 cm). The coefficient of variation (CV) of soil moisture content of 5cm depth over time during the growing season was significantly higher in grassland and shrubland (0.65,0.61) than in forest(0.52), while the CV of 20cm was significantly higher in shrubland and forest (084,0.84)than in grassland(0.72). We found much more interception of rainfall in shrubs (44%) than in forest (27%) and grassland (12%), and believe that the higher interception of rainfall should be responsible for the higher temporal variability of soil moisture content of 5cm depth in shrubland, while the intense soil moisture evaporation may be the major mechanism that contributes to higher temporal variability of soil moisture content of 5cm depth in grassland. Also we find that the net effect of vegetation on soil moisture was significantly greater in grassland than in forest and shrubland, but continuing analysis suggested that water uptake ability provide little contribution to the differences of CV of soil moisture content in both 5cm and 20cm depth. Another interesting finding in this field experiment is that the CV in depth of 20cm was significantly higher than in 5cm, suggesting that the most active area of soil moisture not necessarily happed in the surface.