Abstract: Considering the effect of the slope of reef area, the phenomenon that the noise level in the deep sea area of the South China Sea becomes bigger with depth increasing is analyzed theoretically. The experiment on ambient noise in the deep sea area of the northern South China Sea shows that the noise spectrum level changes little with depth(160 m to 800 m) for most of the time, but at some time, the phenomenon that the noise spectrum level will increase with depth increasing would arise. Considering the effect of topography on sound propagation in the South China Sea, the ambient noise model of deep sea area in the South China Sea is built. When the ship passes the sea around the reef or island, the ship noise spreads into the deep sea sound channel due to the terrain near the reef or island, the distant noise spectrum level increases with depth increasing. The distribution of noise spectrum level with depth can be computed by using the noise model, and it has good agreement with the experimental data. So the variation characteristics of deep sea noise affected by the slope of island reef in the South China Sea are verified.