Abstract: As the second-largest installed capacity hydropower station in the world, a wide and gentle trough valley develops in the Baihetan Hydropower Station reservoir area, however its cause has not been clear.The study on the cause of the valley has essential theoretical and practical significance for research on the evolution of Jinsha River system and the stability evaluation of a resettlement site.The geomorphic characteristics, boundary characteristics, filling sequence, fault subsidence amplitude and development of the Liantang-Longjingwan through valley were studied based on detailed field investigation, ultra-deep borehole logging, wide-area electromagnetic detection, soil radon measurement, and dating technique.The results showed that:(1) The shape of Liantang-Longjingwan through valley was NNW and slightly convex to the east.It was about 8 km long from north to south and 0.8 km wide from east to west, which was formed by fault depression after intersection between Xiaojiang fault and Zemuhe fault;(2) The base of Liantang-Longjingwan valley was formed by Qiaojia fault depression basin, which was covered by debris flows alluvium and diluvium;(3) The fault depression of this valley developed 0.29 million years ago and ended 0.2 million years ago, with a fault depth of 37.5 m.