It is essential to study the sources of suspended particulate organic matter (SPOM) for a comprehensive understanding of the carbon and nitrogen cycles in reservoir located in desert grassland areas. Atmospheric deposition is an important pathway for human and natural nutrients to enter water ecosystems. Previous studies have focused on hotspot areas, but neglected the impact of atmospheric deposition on reservoirs in desert grassland regions with little rainfall, long freezing periods, and a dusty climate. In this study, we measured the contents of organic carbon, total nitrogen, the isotopic composition (delta C-13 and delta N-15), and the ratio of total organic carbon to total nitrogen (C/N ratios) for SPOM in the reservoir and its watershed across both dry and wet seasons using carbon and nitrogen isotope techniques. We also analyzed the sources of SPOM in the reservoir using the end-member mixing model. The results showed that the variation range in SPOM for delta C-13 was - 28.3 to - 21.8 & PTSTHOUSND;; for delta N-15, it was 2.3 to 8.7 & PTSTHOUSND;; and the C/N ratios were 8.12 to 19.12. The variation range of delta C-13 for atmospheric particulate matter was - 20.0 to - 25.0 & PTSTHOUSND;; for delta N-15, it was 2.3 to 11.9 & PTSTHOUSND;; and the C/N ratios ranged from 7.22 to 17.81. The main sources of carbon in the SPOM were atmospheric deposition and terrestrial C3 plants. The origins were significantly different between the wet and dry periods. Atmospheric nitrogen deposition and soil erosion were the primary sources of particulate nitrogen in the reservoir.