Taking 1-year-old Tamarix austromongolica seedlings as objects, an experiment was conducted to study the seasonal variation of root characteristics of T. austromongolica seedlings under different water treatments by artificial trench digging method. The water treatments included dry treatment, moderate irrigation, and sufficient irrigation, respectively. The results showed that water treatments significantly affected the root growth of T. austromongolica seedlings. Depth and growth rate of vertical roots increased with the decreasing of irrigation. Increasing surface irrigation led to root distribution in the upper soil layer and gradual decrease of root extinction coefficient. Root biomass had a significant negative logarithmic relationship with soil depth. The average values of the root/shoot ratios were 0.43, 0.60, 0.90, and 1.12 from July to October, respectively, suggesting an allometric relationship between shoots and roots under different water treatments. Root/shoot ratio increased with decreasing soil moisture content, especially in the later growing season.