Abstract: The fault system of Weixinan Depression in the Beibu-Gulf Basin is very complex. In order to reveal the genetic mechanism, this paper applies sandbox modelling technologies to make a systemic analysis on the formation mechanism of Weixinan Depression fault system on the basis of systematic structural analysis of 3D seismic data. After several physical simulation experiments, the south-north direction experimental model of disharmonic extension can successfully reproduce the complicated fault system of Weixinan Depression, and the major characteristics have clear appearance in this experimental model.The complicated fault system of Weixinan Depression was formed gradually during progressive deformation on the basis of the preexisted basement structure, and the fault system of main rifting phase (Late Eocene-Oliqocene) was formed just by one extension stage (NS trending), but not the result of multi-stage superimposed deformation. This understanding not only can interpretet the fine structure of 3D seismic data in Weixinan Depression, but also can provide a reference for studying the formation and evolution of the peripheal basins of the South China Sea.