A pot experiment was conducted to study the effects of soil water stress in whole growth period on fine root morphology and function of Reaumuria soongorica. The results were as followings:(1) The diameter and volume decreased with the increasing of soil water stress, on the contrary, with the increasing of soil water stress, root length, specific root length(SRL), surface area and specific root surface area(SRA) increased, which indicated that under soil water stress conditions, fine root of R. soongorica seedling could adapt stress by the increasing root length, SRL, surface area and SRA and reducing the diameter and volume. The diameter increased with the rise of root order, root length and SRL increased, while SRA firstly increased and then reduced.(2) Total C content decreased with the increasing of soil water stress, while with soil water stress increasing total N content first decreased significantly and then showed the increasing tendency, which indicated that respiration of fine roots of R. soongorica seedling was restrained under medium soil water condition. Total C content increased with the rise of root order, total N content decreased, which indicated that the low roots have stronger respiration and metabolic activity.(3) The root length had a highly significant positive correlation with C content. The diameter had a significant positive correlation with C content. The SRL was negatively correlated with C content.