Soil moisture is the main restrictive factor for the growth of desert vegetation in the Gurbantunggut Desert. Three soil moisture monitoring sections (include 23 soil moisture monitoring points) were set to research the spatio-temporal variation of soil moisture in a fixed dune at the southern edge of Gurbantunggut Desert in December 2012, two of the monitoring sections were set along the transverse section of dune, the third one which is vertical to the above-mentioned two soil moisture monitoring sections was set in the interdune. The soil moisture monitoring points were located in the interdune, west slope, east slope and top of the dune, respectively. In-situ observation was performed from December 2012 to November 2013 of soil moisture in the 0~400 cm soil layer of the fixed sand dune with a neutron probe, to explore spatio-temporal variation of moisture content in different slope positions of the fixed dune and impact of Haloxylon ammodendron on soil moisture content in the root zone of the plant at its different growth stages. Results show that (1) a 0~400 cm soil profile could be divided into two parts according to average soil moisture content: soil moisture content is higher and vary a lot with the depth in 0~200 cm soil layer, soil moisture content is lower and relatively constant with depth in 200~400 cm soil layer; based on the variation coefficients of soil moisture content, a 0~400 cm soil profile could be divided into 3 layers: active layer (0~40 cm), sub-active layer (40~200 cm) and relatively steady layer (200~400 cm); (2) the difference in soil moisture content was not significant between slope positions, in the west slope, east slope and top of the dune, but it was significant between interdune and the above-mentioned three slope positions (p<0.01), and the soil moisture content in the former was relatively higher; the difference of soil moisture content in different slope positions in active layer, sub-active layer and relatively steady layer consistent with the difference of soil moisture content in the whole profile (0~400 cm); in active layer, soil moisture content in the interdune is 1.25 times, 1.26 times, 1.21 times of the soil moisture content in the west slope, east slope and top of the dune, respectively; in sub-active layer，soil moisture content in the interdune is 1.30 times, 1.36 times, 1.25 times of the soil moisture content in the west slope, east slope and top of the dune, respectively; in relatively steady layer, soil moisture content in the interdune is 1.20 times, 1.22 times, 1.17 times of the soil moisture content in the west slope, east slope and top of the dune, respectively; (3) soil moisture content has obvious temporal variation, the annual variation of soil moisture content could be divided into 3 periods, i.e., moisture gaining period (from March to May), moisture losing period (from June to October) and moisture stable period (from November to February the following year); Soil moisture recharge is more than discharge in moisture gaining period and less than discharge in moisture losing period, soil moisture recharge and discharge are almost same in moisture stable period; (4) soil moisture content was much lower in autumn than in spring and summer in the root zone of the Haloxylon ammodendron of different growth stages; was quite low in all the four seasons in the root zone of grown-up Haloxylon ammodendron; varied sharply between seasons in the root zone of young Haloxylon ammodendron and between the grown-up plants and the young plants in spring and summer.