Nitraria dunes can fix a huge amount of moving sand and are one of the most effective ways to protect oasis from moving sand in Northwest China. In recent decades, Nitraria dunes in Northwest China have seriously degraded. As a result, the fixed sand is released, and the oasis is damaged again and again. How to sustain Nitraria dunes long term and prevent the Nitraria plant from dying is an important issue. Based on observation over time, we hypothesized that declining soil water content resulted from an imbalanced soil water budget leading to Nitraria dunes degradation. However this hypothesis hasn't been proved satisfactorily for the lack of long-time field experiments. To prove this hypothesis, we observed soil water content from January 2008 to June 2012 in different degradation stages of Nitraria dunes in the marginal oasis of Minqin, Gansu Province. The methods we used to observe soil water content were the neutron moisture gauge method and weighting method after dried. Results showed that average volumetric soil water content was the highest in 2008 and the lowest in 2010.The variation of volumetric soil water content was the most obvious at the depth of 0-50 cm.The vertical variation pattern of volumetric soil water content was different in different degradation stages. At the depth of 0-90 cm, volumetric soil water content varied the greatest in the early growth stage and rapid growth stage for low water-holding capacity, high seepage power and high air permeability of soil. Three kinds of soil layers divided by soil water variation features occurred in these stages of the Nitraria dunes. These were sudden-change layer, stable layer and increasing layer. However, in the peak growth stage and senescence of Nitraria dunes, volumetric soil water content varied slightly because of crust on the Nitraria dunes surface. Average volumetric soil water content was lower in the early spring, gradually rising in the summer and declining in the autumn.The maximum soil water content occurred in July and September. Moreover, the variation of volumetric soil water content in different seasons was the most obvious at the depth of 0-30 cm. With increasing depth, the volumetric soil water content variation became less. During different degradation stages, the volumetric soil water content in the early growth stage was the highest because precipitation was easy to infiltrate. However, the soil water content was the lowest in the peak growth stage and senescence because precipitation cannot infiltrate without a significant precipitation event. And soil water content in the peak growth stage and senescence of Nitraria dunes was lower than their wilting coefficient. As a result, soil water content cannot satisfy the basic water demand for plant growth which was the principle reason why Nitraria dunes degraded. Through this study, it can be concluded that declining soil water content is the main reason why Nitraria dunes degraded. To take advantage of precipitation efficiently and prevent evapotranspiration on the surface of Nitraria dunes is the key to protecting Nitraria dunes and sand stabilization. This result will provide a positive influence on subsequent vegetation sand-control activities.