Property of root distribution and its relation to spatial and temporal patterns of root water uptake were the basis to establish efficient water management strategies for fast-growing and high-yield triploid Populus tomentosa pulp plantation, which should be fully understood. In this study, the spatial distribution pattern of roots of poplar trees growing under the wide-and-narrow row spacing scheme was studied with profiling method in a 5a triploid P.tomentosa pulp plantation, located in Gaotang county, Shandong province. 1620 soil cores of 2000 cm³ volume were taken out around 8 trees. The water contents of soil profiles were observed consecutively for four months with a time domain reflectometry (TDR). The rates of water uptake by roots were calculated based on field measurements. The relationship among soil water contents, distribution of roots, and patterns of water uptake by roots were analyzed to see how root water uptake responded to fine root (< 2 mm) distribution and changes in soil water availability. The results showed that the average root length densities (RLD) of poplar trees at both sides of planting belts and at different radial distances were close (P>0.05). However, RLD varied significantly among different soil horizons (P<0.01). Roots were mainly distributed in soil layers at depths of 0 20 cm and 60 150 cm. Roots in those two layers accounted for 29% and 57% of the total length density of roots respectively. The RLDs of trees in different root diameter classes were significantly different (P<0.01) and the ratio of roots varied with spatial locations. However, in the root system of tiploid P.tomentosa, about 44% of the roots corresponded to a diameter of 0.2 0.5 mm, followed by 35% of roots of 0 0.2 mm diameter, a 15% of roots 0.5 1 mm and only a 6% of roots greater than 1 mm. Among different orientations, the distribution of roots was shallower at east side than that of west side. The roots of 0 0.5mm within 240 280 cm radial distances at the east side were also more abundant compared with the west side. Therefore, under the wide-and-narrow row spacing planting scheme, the soil depth and root diameter class are two main factors affecting the distribution of roots of poplar trees while planting orientation can influence their morphology and architecture. The water uptake by roots is primarily affected by the spatial distribution of fine roots but will change along with the availability of water in soil profile. When the availability of water increases in top soil, the water uptake by roots mainly happens in top soil. The contribution of roots in deep soil layers will increase when the availability of water in top soil becomes low. If the water condition of a soil profile is highly heterogeneous, the water uptake by roots mainly occurred in zones with high fine roots length densities and availability of water. On the other hand, water uptake is in good agreement with the distribution of fine roots when water spreads evenly in soil profiles and the water stress is not imminent.