Abstract: A fault lies across in a hydropower reservoir region only 3 km upstream of Majiahe dam on the Jinsha River Its structure patterns and deformation features vary greatly between different classes of fracture surfaces and even different parts of the primary fracture surfaces.Therefore,characterization of the fault is the key to stability assessment of the dam as well,which has been a hot topic for debating.Based on new data collected by large-scale geological mapping,surveying adits and structure analysis,the authors conclude that that fault is a small thrust in the interior area of a plateform.It is the first time that the geometry and kinematics features as well as influence region are determined.The dynamics of the fault formation is also analyzed.The research results provide information needed for construction and safe operation of the dam.