<p>Based on high-quality mooring data collected near the Dongsha Islands in the northern South China Sea in 2016, this study employed a multi-scale analysis method to systematically investigate the spatiotemporal evolution characteristics of internal waves and the nonlinear interactions between diurnal internal tides and near-inertial waves. The results revealed pronounced internal tides and near-inertial motions during the observation period, with diurnal internal tides exhibiting stronger energy than both semidiurnal tides and near-inertial waves. Notably, near-inertial energy decreased significantly with depth. The coupled <i>D</i>₁–<i>f</i> spectral peaks indicated wave-wave interactions between near-inertial waves and diurnal internal tides. Diurnal tides were primarily dominated by low vertical modes, whereas near-inertial waves and <i>D</i>₁–<i>f</i> coupled waves exhibited distinct high-mode structures. Additionally, near-inertial waves predominantly propagated with downward phase, while the <i>D</i>₁–<i>f</i> waves mainly propagated with upward phase. At the observation site, the analysis identified a strongly nonlinear three-wave resonance phenomenon consistent with parametric subharmonic instability (PSI), in which high-frequency diurnal tides acted as parent waves, and low-frequency near-inertial waves and <i>D</i>₁–<i>f</i> coupled waves served as daughter waves.</p>