Based on the data recorded by the permanent seismic stations in Guangdong Province and Hong Kong and by portable seismic stations on the Dangan Islands, the authors obtained a series of laterally homogeneous 2-D models using Pg and PmP phases for every station, determined the crustal average velocity, and constrained variations of Moho depth. The results indicate the onshore-offshore transitional zone is the transitional boundary between the northern South China Sea and South China. The results have improved the understanding on crustal structure and tectonics of South China and the northern South China Sea.