Based on the data observed at Ejin Banner from June to July in 2006 and 2007, using fractal theory and statistical method, the relationship between root distribution of Populus euphratica oliv and soil moisture content of root system was analyzed. A new definition, expectation of soil moisture content (E/ cm·g·g^-1), was introduced in order to reflect better characteristic of soil moisture in root zone. The function between expectation of soil moisture content and root fractal dimensions was obtained. Also built the preliminary root water uptake model, and a good result by comparing the simulation value with observation value was achieved.