Abstract: Fixed ammonium is commonly defined as the ammonium ion not replaced by neutral salts, particularly between the layers of 2:1 clay mineral. It is also an important component of soil nitrogen pool, and plays a significant role in plant growth. However, the content of fixed ammonium in aeolian sandy soil and its influencing factors are largely unexplored, which limit understanding of the formation of soil fertility and nitrogen cycle in sandy lands. In this study, soil fixed ammonium was determined in three sampling plots (bare sandy land, Salix psammophila land, and Artemisia ordosica land) in the Mu Us Sandy Land, and further to examine the effects of soil particle size and mineral composition on fixed ammonium. In June 2019, ten 5 m × 5 m subplots were randomly selected in each sampling plot. In each subplot, ten soil cores at 0~20 cm depths were randomly collected, and mixed to create one composite sample. All soil samples were air-dried and divided into three fractions (> 0.25 mm, 0.1-0.25 mm, < 0.1 mm). The contents of fixed ammonium in all soil fractions were measured by the Silva-Bremner method. Soil total nitrogen was analyzed using a vario EL III elemental analyzer (Elementar, Germany). The soil mineral composition was determined by the X-ray diffraction (D8 Advance, Bruker Biospin, Germany). The results showed that the average content of soil fixed ammonium in the research site was 18.63 mg/kg, accounting for 8.77% of the soil nitrogen pool, where the soil fixed ammonium and total nitrogen content were most distributed in the Artemisia ordosica land (23.03 ± 1.88 mg/kg and 274.71 ± 8.38 mg/kg, respectively), followed by the Salix psammophila land (16.82 ± 1.25 mg/kg and 197.15 ± 6.74 mg/kg, respectively), and the least in the bare sandy land (16.63 ± 0.61 mg/kg and 145.99 ± 9.45 mg/kg, respectively). The composition of soil particle significantly differed among three different plots, with more fine-textured soils in Artemisia ordosica land than that in Salix psammophila land and bare sandy land. In addition, the content of fine-textured soils in Salix psammophila was higher than that in bare sandy land. Soil particle composition was significantly correlated to fixed ammonium, indicating the finer the soil particle size, the higher the fixed ammonium content. Seven dominant minerals were found in the three sampling plots soil, including quartz, calcite, plagioclase, orthoclase, albite, anorthitic, and calcium aluminum silicate. Three sampling plots were characterized by different soil mineral composition. Specifically, Artemisia ordosica land had the greatest content of calcite, and orthoclase; Salix psammophila land had the greatest quartz content; and the bare sandy land showed the highest plagioclase content. The relationship between soil mineral composition and fixed ammonium was not significant. It infers that that soil fixed ammonium in the Mu Us Sandy Land largely depended on the soil physical composition rather than the mineral composition. The vegetation primarily affected soil fixed ammonium by changing soil particle composition, suggesting that the increase of soil fine-grained matter via vegetation rehabilitation can contribute to the content of fixed ammonium. The findings can offer a significant theoretical support to land desertification control, and further to increase the biological productivity.