Aims It is more difficult to determine soil moisture in root zones (RSM) than soil moisture in the top-most layer (TSM). The relationship between TSM and RSM based on point A is useful for acquiring RSM at point B from its TSM if the relationship between TSM and RSM is general. The general relationships established so far have been based on only one crop or one ecosystem. Our aims are to determine the general relationship between TSM and RSM in China over various crops and ecosystems and to explore the effects of ecosystem, soil property, precipita- tion, vegetation, soil thickness and the order of magnitude of soil moisture on the relationship. Methods We used 3 437 pairs of TSM and DSM data from 31 stations at 109 observation sites. The data repre- sent all the terrestrial ecosystems, including forest, grass, agriculture, desert and mire, in 2006 from the China Ecosystem Research Network. We used linear regression in mathematics and effect analysis in physics to study the general relationship between TSM and RSM and to determine the influence of ecosystem, soil property, pre- cipitation, vegetation, soil thickness, the order of magnitude of soil moisture on the relationship. The data are di- vided into calibration and verification groups for producing the relationship. Important findings There is generally a linear relationship between TSM and RSM. The coefficient of determi- nation (R 2 ) between RSM and TSM is high for forest and mire ecosystems (R 2 > 0.79 for both calibration and verification), intermediate for agricultural (R 2 > 0.80 for calibration and 0.70 for verification) and grass