Abstract: The Taiwan Strait Tunnel(T. S.T)，as a great project，has been proposed for a long time and met many challenges from the beginning of the feasibility study that has attracted attentions among scientists in both sides of the Taiwan Strait The key question is whether this undersea tunnel could he constructed with present technologies under such complex geologic and seismologic conditions. Results of current research indicate that sea floor in the north of the Strait along Coast of Taoyuan county of Taiwan is covered by the late Tertiary to Quaternary sandy shale and shale with a thickness of more than 300m without large fault-belt，which could serve as a good bearing and impermeable layer Along the coast of Ping tan Island，Fujian in the west of the Strait，there is an area with vast distribution of granite。which is ideaI for the construction of the tunnel termina1. We therefore suggest that the shortest line with a distance of 126km from Pingtan Island ，Fujian to Hsinchu，Taiwan should he considered as the undersea tunnel1ine for its simple and stable geological condition.