The existing genetic models of the South China Sea (SCS) include an extrusion model of the Indochina Peninsula, a back-arc extension model, and a subduction and dragging model of the Proto-South China Sea (PSCS). However, none of these models has been universally accepted because they do not fully match a large number of geological phenomena and facts. By examining the regional tectonics and integrating them with measured data for the SCS, in this study, a back-arc spreading-sinistral shear model is proposed. It is suggested that the SCS is a back-arc basin formed by northward subduction of the PSCS and its formation was triggered by left-lateral strike-slip motion due to the northward drift of the Philippine Sea Plate. The left-lateral strike-slip fault on the western margin caused by the Indo-Eurasian collision changed the direction of the Southwest Sub-basin’s spreading axis from nearly E–W to NE–SW, and subduction retreat caused the spreading ridge to jump southward. This study summarizes the evolution of the SCS and adjacent regions since the Late Mesozoic.