Maize–mustard rotation dominates sloping purple farmland in the Three Gorges Reservoir Area, where nitrogen (N) and phosphorus (P) losses jointly threaten reservoir water quality. This paper presents the P component of a combined N–P study and shares the same in situ runoff-plot platform as its parallel N migration study. A two-year in situ runoff-plot campaign at the National Purple Soil Fertility and Fertilizer Efficiency Monitoring Base quantified 0–60 cm soil P migration patterns across four rainfall regimes (TR, HR, MR, LR) and four growth stages (CPS, SFS, MPS, WFS). Results showed that rainfall runoff drove P migration both horizontally and vertically. TR and HR events, which accounted for only 6.05% of rainfall events, produced 73.26% of annual surface P loss (1.076 kg ha⁻¹ yr⁻¹) and 58.85% of annual subsurface leaching (0.440 kg ha⁻¹ yr⁻¹); over 78% of the total P load occurred during the maize planting stage. Soil depth dominated transport attenuation: PP fell from 93.42% at the surface (L₀) to 52.57% at 40–60 cm (L₃), and the 20–60 cm subsoil produced a net reduction of 83.33% in downward P flux leached from the topsoil, lowering the risk of P migration to deeper layers. Combined with the companion N study, both elements share extreme-rainfall drivers and the maize-stage critical loss window but diverge vertically: nitrate N shows stronger deep mobility, while the subsoil buffers P as a stable sink. This coupled N–P evidence supports site-specific control of agricultural non-point source pollution.