The relationship between fluid density and saturation and sonic wave velocity of rock samples taken from the WXS Depression in the South China Sea was studied by an oil-water replacement experiment under simulated in-situ temperature and pressure conditions. Two kinds of low-density oils (0.691 and 0.749 g/cm³) and two kinds of high-density oils (0.834 and 0.873 g/cm³) were used to saturate the rock samples at different oil-saturation states, and the saturated P- and S-wave velocities were measured. Through Gassmann’s equation, the theoretical P- and S-wave velocities were also calculated by the fl uid replacement method. With the comparison of the measured values and the theoretical values, this study comes to the following conclusions. (1) With the increase of oil saturation and the decrease of water saturation, the P-wave velocity of rock samples saturated by low-density oil increases and the changing rule is in accord with the effective fl uid theory; the P-wave velocity of rock samples saturated by high-density oil decreases and the changing rule goes against the theory. (2) With the increase of oil density (namely 0.691→0.749→0.834→0.873 g/cm³) when oil saturation is unchanged, P-wave velocity increases gradually. (3) The S-wave velocity is always stable and is not affected by the change of oil density and saturation. The results can be used to constrain pre-stack seismic inversion, and the variation rule of sonic wave velocity is valuable for hydrocarbon identifi cation in the study area.