[Objective] In order to grasp the current situation and spatial variability pattern of black soil layer thickness in the typical black soil region within a small watershed, and realize dynamic monitoring of black soil layer thickness in such areas. [Methods] This study selected a small watershed of Heshan Farm in Nenjiang City, Heilongjiang Province. Soil sampling points were uniformly distributed at intervals of 100 meters, and undisturbed soil samples were collected from surface to parent material using soil augers to determine the thickness of the black soil layer. Meanwhile, terrain and land use data were extracted based on UAV aerial photography data. The study analyzed the thickness of black soil layer in the small watershed and its influencing factors. [Results] Among the 295 sampling points in the small watershed, the thickness of black soil layer varied from 0 to 160 cm, with an average of 44.1 cm. "Popihuang" (black soil layer thickness ≤20 cm) accounted for 17.63%, thin black soil (<30 cm) comprised 43.39%, medium layer (30—60 cm) constituted 36.27%, and thick layer (>60 cm) accounted for 20.34%. Thick layer black soil was primarily distributed in the lower slopes, in the vicinity of gullies, and other gently sloping areas of the watershed, while thin layer black soil was predominantly found in steep slopes and areas with sandy gravel parent material, and medium layer black soil was predominant in other regions. Terrain indices such as topographic position index, curvature, slope exhibited significant correlations with the black soil layer thickness. The thickness of black soil layers in cultivated land was significantly lower compared to grassland and forest land. [Conclusion] The thickness of black soil layer exhibits significant spatial heterogeneity within the small watershed, which is primarily influenced by terrain characteristic, parent material and landuse. The research findings can be used as one of the bases for dynamic monitoring of regional black soil layer thickness, and can provide data support for the formulation of soil and water conservation measures in small watershed.