Abstract: The Nansha Islands are located in the southern part of the South China Sea. Its geographical location is very important to China's maritime rights, development strategy, and national security. It is of great significance to systematically understand the Nansha Islands, fully understand the topography of the South China Sea, ensure the military security of the South China Sea and safeguard the national maritime rights and interests to carry out the research on the monitoring and change of the water depth and topography of the Nansha Islands. In this study, combined with Sentinel-2 and ICESat-2 (ice, cloud, land elevation satellite-2)data, the active-passive fusion sounding algorithm is used to invert the topography of the shallow sea area of typical islands and reefs such as Bai Jiao and Beizi Dao in the Nansha Islands of the South China Sea. The findings are as follows: 1) A high-precision model for the shallow water depth of typical islands and reefs in the Nansha Islands based on multi-source satellite remote sensing inversion has been established (R2>0.9, MAE(mean absolute error)<0.4m, RMSE(root mean squared error)<0.7m), which is proven to have high stability and reliability. 2) From 2016 to 2024, significant topographic and spatial structural changes occurred on Bai Jiao and Beizi Dao, with Bai Jiao experiencing an expansion of the navigation channel and an increase in land area due to land reclamation projects. The beach area of Beizi Dao showed a relationship of this growth and reduction with the Bai Jiao flat area, while the reef front slope area exhibited certain annual fluctuations. 3) The significant changes in the topography of the Bai Jiao are closely related to the implementation of land reclamation projects, which are mainly affected by human factors. The change of Beizi Island is mainly affected by natural factors, including monsoon and typhoon activities, as well as seawater erosion and sediment redistribution. These natural processes and extreme weather events jointly shape the geographical morphology of Beizi Dao.This study has significant reference value for efficiently detecting the water depth and topography of typical islands and reefs in China using multi-source satellite remote sensing means and can provide important technical support for China's long-term monitoring of the topographic and temporal-spatial changes in shallow water areas such as the Nansha Islands.

Key words: South China Sea islands, Sentinel-2, ICESat-2, Bathymetric Inversion, space-time monitoring