Abstract: The characteristics of internal waves and acoustic-energy fluctuations are analyzed by the experimental data from the coast of Hainan Island in China in June 2020. Internal waves with a diurnal tide period dominate the experimental sea area, accompanied by high-frequency internal-wave activities. Owing to internal-wave activities, the peak-to-peak value of the energy fluctuations of 360 Hz continuous wave signals at 20 km exceeds 30 dB and that of 320–400 Hz linear frequency modulation signals exceeds 15 dB. The reasons for different fluctuation characteristics of continuous wave signals and linear frequency-modulated signals are discussed based on the experimental measurements and numerical simulation of acoustic field. The results show that the internal-wave activities in the experimental sea area can move the interference stripes between acoustic normal modes, leading to the rapid fluctuations of sound energy at a specific frequency at the receiving location. The energy fluctuations of broadband signals are much smaller than those of single-frequency signals because of the averaging effect of bandwidth. When the propagation velocity of internal waves changes, the time and position of internal-wave components along the sound propagation path change, varying the time of rapid fluctuations in the sound field accordingly.