[Objective] The relationships between soil carbon and nitrogen in a farmland ecosystem and soil physical and chemical properties were analyzed to provide a theoretical reference for the rational utilization of land resources in an alpine region.[Methods] Nineteen representative farmland sample plots were selected on a southern slope of the middle section of the Qilian Mountains. Total carbon (TC), total nitrogen content (TN), organic carbon content (SOC), and water content (SWC) of soils in the study plots were measured in the laboratory, and soil bulk density (ρ_b), particle size (clay, silt, sand), pH value, and other physical and chemical properties were determined.[Results] ① TC and TN decreased with increasing soil depth, and the average contents of TC and TN were 35.47 g/kg and 2.41 g/kg, respectively. ② The contents of SOC, SWC, clay, and silt also decreased with increasing soil depth, and the contents of soil ρ_b, pH value, and sand increased. ③ Soil physical and chemical properties were related to each other and jointly affected soil carbon and nitrogen content. The direct interaction between soil carbon and nitrogen was significant. Soil ρ_b had a direct effect on soil TC. SWC had an indirect effect on soil TC by affecting soil TN. Soil silt and clay contents had a direct effect on soil TN. Soil pH value had an indirect effect on soil TN by affecting soil silt and clay contents.[Conclusion] Farmland soil on a southern slope of the Qilian Mountains was relatively fertile, and TN and organic matter content were at a high level that could provide sufficient soil nutrients for vegetation growth in the study area. Under the farming practices currently used, the increase in soil TN and SWC was conducive to the accumulation of soil TC content, and the increase of soil TC, clay, and SWC content was conducive to the accumulation of soil TN content.