Acoustic ducting effects observed in a nonlinear internal wave field on the shelf of the South China Sea northeast of Tung-Sha Island (water depth of 6120 m) are investigated. Environmental measurements from stationary instruments in the area show that the nonlinear internal waves have large amplitudes (over 40 m) and strong curvature. A principal component interpolation method is used to reconstruct the time-dependent three-dimensional internal wave field. A three-dimensional parabolic equation model is then employed to simulate monochromatic 400-Hz acoustic propagation parallel to the internal wave fronts for comparison to field data. The passing internal waves alternately elevate and depress the thermocline to form strong three-dimensional near-bottom acoustic ducts between the wave troughs and strong surface ducts of mixed layer water in the wave troughs. These alternating, depth-dependent acoustic ducts create asynchronous, vertically-bifurcated sweeping beams of ducted acoustic energy. The three-dimensional acoustic propagation modeling results are shown to mimic the field observations to a strong degree and elucidate the propagation physics in this highly complex environment.