Abstract: The vertical directionality of ocean ambient noise is one of the typical spatial characteristics of sonar background. It can not only reflect the spatial characteristics and propagation characteristics of the noise source, but also can be used to invert the characteristic parameters of the ocean environment. The vertical directionality of ocean ambient noise in the South China Sea waters with island and reefs is analyzed according to the measured noise data in field, and the results show that the main lobe of the vertical directionality pattern of low-frequency ocean ambient noise is concentrated in the angle range from -30° to 30° when the receiving array is far away from the shipping lane, and there is no notch in the horizontal direction. A simplified ship noise source model in shipping lane areas is obtained from the AIS ship distribution data, and the 3D ray algorithm of sound propagation combined with the topographic data in the island and reef area is used to calculate the vertical directionality of low-frequency ocean ambient noise. The calculation results are consistent with the analytical results of the measured data, which indicates that when the receiving array is far away from the shipping lane, the low-frequency ocean ambient noise is incident to the vertical receiving array in a nearly horizontal direction. The research results can provide support for improving sonar performance in the areas far away from shipping lanes.