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(1) Background: Water transport systems play an important role in maintaining plant growth and development. The plasticity responses of the xylem anatomical traits of different species to the environment are different. Studies have shown that there are annual growth rings in the secondary root xylem of perennial herbaceous species. Studies on xylem anatomical traits, however, have mainly focused on woody species, with little attention given to herbaceous species. (2) Methods: We set 14 sampling sites along a rainfall gradient in arid and semiarid regions, and collected the main roots of native (<i>Potentilla</i>) and non-native (<i>Medicago</i>) perennial forbs. The xylem anatomical traits of the plant roots were obtained by paraffin section, and the relationships between the xylem traits of forbs were analyzed by a Pearson correlation. (3) Results: In the fixed measurement area (850 μm × 850 μm), the vessel number (NV) of <i>Potentilla</i> species was higher than that of <i>Medicago</i> species, while the hydraulic diameter (Dh) and mean vessel area (MVA) of <i>Potentilla</i> species were lower than those of <i>Medicago</i> species. With the increase in precipitation along the rainfall gradient, the Dh (R² = 0.403, <i>p</i> = 0.03) and MVA (R² = 0.489, <i>p</i> = 0.01) of <i>Medicago</i> species increased significantly, and NV (R² = 0.252, <i>p</i> = 0.09) decreased, while the hydraulic traits of <i>Potentilla</i> species showed no significant trend with regard to the rainfall gradient. (4) Conclusions: The hydraulic efficiency of non-native <i>Medicago</i> forbs was higher than that of native <i>Potentilla</i> forbs, and the hydraulic safety of native <i>Potentilla</i> forbs was higher than that of non-native <i>Medicago</i> forbs. With the decrease in precipitation, the hydraulic strategies of non-native <i>Medicago</i> forbs changed from efficiency to safety, while native <i>Potentilla</i> forbs were not sensitive to variations in precipitation.