Large-amplitude nonlinear internal waves (NLIWs) are prevalent in the South China Sea (SCS), causing significant energy dissipation and sediment resuspension on the continental shelf. However, the seasonal variations and sediment dynamics of NLIWs, driven by the SCS’s complex multiscale processes, remain poorly understood. Using year-round mooring observations, this study investigates the seasonal spatiotemporal variations, instability structures, and sediment resuspension of NLIWs. Three distinct NLIW types are identified: Type-A and type-B NLIWs originate from consecutive ebb and flood phases under diurnal-dominant barotropic tides near the Luzon Strait, while type-C NLIWs arise from flood phase under semidiurnal-dominant tides. Within single type-A wave clusters, their daily arrival interval varies markedly: It initially shortens (<24 h) before lengthening (≈26 h), while that of type-B and type-C NLIWs remain relatively stable and phase locked to the barotropic tide. These variations are governed by nonlinear dynamics and are reproduced by an across-basin simulation. The occurrence and amplitude of NLIWs at the mooring site exhibit intraseasonal and seasonal variabilities, driven by meridional wave front displacements that are primarily regulated by mesoscale eddies and Kuroshio intrusions. As NLIWs shoal, vortical structures form at their trailing edges, reflecting nonlinear evolution and sustaining sediment resuspension that scales linearly with wave amplitude. Specifically, large-amplitude type-A waves, accompanied by pronounced trailing vortices, induce more turbid (0.4 mg L⁻¹) and longer-lasting (70 min) resuspension than type-B and type-C NLIWs, which generate weaker (0.3 mg L⁻¹) and shorter (20–30 min) resuspension. These findings enhance our understanding of the spatiotemporal transitions and sediment dynamics of NLIWs.