(1) Drought and flooding are contrasting abiotic stressors for plants. Evidence is accumulating for root anatomical traits being essential for the adaptation to drought or flooding. However, an integrated approach to comprehensively understand root anatomical traits has not yet been established. (2) Here we analysed the root anatomical traits of 18 wild Poaceae species differing in adaptation to a range of soil water content. Regression model analyses revealed the optimal anatomical traits that were required by the plants to adapt to low or high soil water content. (3) While the area and number of each root tissue (e.g. stele, cortex, xylem or aerenchyma) were not strongly correlated to the soil water content, the ratio of the root tissue areas [cortex to stele ratio (CSR), xylem to stele ratio (XSR) and aerenchyma to cortex ratio (ACR)] could fully explain the adaptations of the wild Poaceae species to the soil water gradients. (4) Our results demonstrate that the optimal anatomical traits for the adaptations to soil water content can be determined by three indices (i.e. CSR, XSR and ACR), and thus we propose that these root anatomical indices can be used to improve the tolerance of crops to drought and flooding stresses.