| Summary: | Soil moisture (<i>SM</i>) significantly affects the exchange of land surface energy and the stability of terrestrial ecosystems. Although some conclusions have been drawn about the effects of <i>SM</i> on the ecosystem water use efficiency (<i>WUE</i>), the influence mechanism and the quantitative assessment framework of <i>SM</i> on <i>WUE</i> are still unclear. This study provides an analysis framework for the feedback relationship between <i>SM</i> and <i>WUE</i> based on the dependence of the evaporation fraction on <i>SM</i> and output datasets from remote sensing and the Global Land Data Assimilation System. The results show that the range of <i>WUE</i> of terrestrial ecosystems of China was 0.02–19.26 g C/kg H<sub>2</sub>O in the growing season with an average value of 1.05 g C/kg H<sub>2</sub>O. They also show a downward trend in 43.99% of the total area. In the evapotranspiration (<i>ET</i>) pathway, SM negatively affected <i>WUE</i>, and the sensitivity coefficient ranged from −18.49 to −0.04. In the net primary production (<i>NPP</i>) pathway, the sensitivity coefficient ranged from −68.66 to 43.19. Under the dual effects of the <i>ET</i> and <i>NPP</i> pathways, the influence of <i>SM</i> on <i>WUE</i> was negative in 84.62% of the area. Variation in <i>SM</i> led to significant <i>WUE</i> variability. Generally, the percentage change in <i>WUE</i> (Δ<i>WUE</i>) ranged from 0% to 190.86%, with an average value of 28.02%. The maximum Δ<i>WUE</i> ranged from 0% to 758.78%, with an average value of 109.29%. The <i>WUE</i> of forest ecosystems showed strong resistance to <i>SM</i> variation, whereas that of non-forest vegetation was more sensitive to <i>SM</i> variation. This analytical framework provides a new perspective on the feedback relationship between <i>WUE</i> and <i>SM</i> in terrestrial ecosystems.
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