Abstract: Excess nitrogen is accumulated in the vadose zone because of the overuse of nitrogen fertilizer. The Baiyangdian Watershed is located in the western part of the North China Plain and has been impacted by overexploitation of groundwater, resulting in decreased groundwater levels and increased vadose zone thickness. However, at a large spatial scale, the mechanism and controlling factors of nitrogen leaching and transformation affected by different geomorphic types and soil texture remain unclear. In this study, we selected 9 deep soil profiles (20 m depth) through two typical alluvial-proluvial fan systems (including the loessal terrace, alluvial-proluvial fan, flood plain, and lake depressions) in the Baiyangdian Watershed to investigate the accumulation, leaching, and transformation processes of nitrate-nitrogen (NO₃⁻-N) via hydrochemical analysis and the chloride ion balance method. Results show: 1) The soil texture within a soil profile, which is influenced by geomorphic types, exhibits spatial heterogeneity. Specifically, the alluvial-proluvial fan is characterized by a sandy texture, while the flood plain exhibits a clayey texture. These differences in soil texture play a crucial role in determining the spatial distribution of NO₃⁻-N accumulation and leaching within the vadose zone. The NO₃⁻-N accumulation ranges from 352.7 to 3 276.7 kg·hm⁻² in the soil profiles from 0 to 20 m, with the maximum accumulation occurring in the lake depressions. Conversely, NO₃⁻-N leaching below the root zone ranges from 9.8 to 252.0 kg·hm⁻²·a⁻¹, with the maximum leaching occurring in alluvial-proluvial fan. The NO₃⁻-N accumulation shows increasing trend while NO₃⁻-N leaching shows decreasing trend from upstream to downstream in soil profiles controlled by the same geomorphic types. The nitrogen accumulation of orchard (1 544.0–3 133.8 kg·hm⁻²) is higher than that of vegetables (2 641.6 kg·hm⁻²) and wheat-maize/wheat-soybean rotation (352.7–3 276.7 kg·hm⁻²) among different land use types. 2) The transformation of nitrogen in the deep vadose zone of the Baiyangdian Watershed is mainly influenced by crop absorption, nitrification, and denitrification. Nitrification occurred in the entire soil profile. The variation in the peak value in the silty loam soil of the alluvial-proluvial fan area was caused by the joint effect of nitrification and denitrification. However, denitrification occurred mainly in 0–10 m soil layer in the flood plain or lake depressions of clay-textured layers because of the decrease in soil organic carbon from the surface to the deep vadose zone. This research not only advances current research on nitrate migration and transformation from point scale to watershed scale but also provides valuable insights for future investigations on the physical and geochemical processes of nitrogen leaching at a watershed scale. Additionally, this study provides scientific support for the control of agricultural non-point source pollution, protection of the aquatic environment, and integration of land use management in the Baiyangdian Watershed.