Abstract: Slab windows are generated from the subducting plate, but not necessarily on the subducting ridge. However, slab windows related to the ocean ridge are taken great proportions in the world. The young subduction ridge, which is usually less than 10 Ma, is much hotter and has positive buoyancy, so there is wide gap between two sides of subducting ridges. The fossil ridge of South China Sea has subducted under the Philippine Sea plate along the Manila Trench. There are double chains of volcanoes in the eastern edge of the South China Sea(Luzon), where the west volcanic chain has stopped eruption since Pliocene, but the east one has a lot of volcanic activities up to now. The two volcanic chains are separated at 20°N, and the east volcanic chain has extinguished at 17.8°N, while the west one has extended to the island of Mindoro. So the volcanic gap between volcanic chains suggest the slab window below Luzon, which was formed by the extinct ridge subduction. Moreover, the characteristics of time and space about adakites after late Holocene also implied that the slab window existed between 17°N and 17.5°N in the Luzon Island.