The study used photomicrographic technique to compare the morphological characteristics and quantity characteristics between the root and stem vessel elements of Tetraena mongolica, and used the SMA to analyze the allometric relation between the length and diameter of vessel elements, in order to discuss the adaptability of secondary xylem elements of T. mongolica to the grassland desertification area. The results showed that: (1) the root and stem vessel elements of T. mongolica are mainly pitted and reticulate catheter types, and both have no tail, and both of them have simple perforation plate; (2) the root and stem vessel elements of T. mongolica which catheter length are short grade level, and their catheter diameter are small short level and both have wide and narrow vessel elements, and stem vessel elements are more “miniaturization” than that of roots; (3) regardless of the roots or stems, their length and diameter of the catheter molecules exhibit allometric relationship, and the growth rate of vessel length is greater than that of diameter. These results are helpful to understand the adaptation strategies from catheter scale in desert area of steppe desert region.