The authors analyze the internal wave spectra of the ADCP（Acoustic Doppler Current Profiler） data from the South China Sea Monsoon EXperiment (SCSMEX) in 1998. The results suggest that the level of nonlinear interactions amongst internal waves affects the characteristics of internal wave spectra in the north South China Sea. In the seasonal thermocline (depths of 22 m and 58 m), the falloff rate of the spectra with frequency σ trends to fall between σ–1 and σ–2 for internal-wave band due to strong nonlinear interactions; furthermore, tidal harmonic frequency peaks with high energy show an amplitude decrease of σ–3 with frequency. Below the seasonal thermocline (depth 130 m), the nonlinear interactions are weakened with increasing depth so that the internal wave spectrum is similar to canonical Garrett-Munk spectrum, both showing a falloff rate of σ–2.