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The destroyed vegetation dominated by Haloxylon ammodendron (C. A. Mey.) Bunge resulted in more moving sand dunes in Gurbantunggut Desert of Northwest China. Natural vegetation restoration is difficult due to high seedling mortality of H. ammodendron, which has unclear causes. Two independent field experiments on relationships between key environmental factors and survival of H. ammodendron seedlings were carried out in Gurbantunggut in the spring of 2008 and the summer of 2009. The spring experiment suggested that the number of ephemeral plants was significantly increased with more precipitation, and they competed for water and reduced survival of H. ammodendron seedlings. The highest H. ammodendron seedling survival was for treatment with least precipitation, though the differences were not significant among treatments. The summer experiment showed that variation in seedling mortality was well explained by water content in 0-20 cm layer. It is concluded that in spring, competition for soil water from ephemeral plants is the primary determinant of seedling survival. In summer, without competition of ephemeral plants, seedling survival was mainly determined by drought, which can be either drought of the soil or dryness of the air: when the soil moisture content at upper layer was <1.25%, dryness of the air governed the seedling mortality, otherwise the dryness of the soil governed the seedling mortality.