Abstract: Through an analysis of the setting and basic characteristics of the tectonic deformation of the Tertiary Yinggehai basin in the northwestern South China Sea,the characteristics and distribution pattern of the fault system in the basin were studied in details,and on that basis,combined with a study of the tec-tonic evolution in the area,the genetic mechanism of the fault was studied.The NW-trending fault con-trolled the general tectonic framework of the basin,and especially the strong dextral shear formed the N-S-trending echelon tensile fractures and induced the occurrence of the mud-fluid diapirism.Then the mud-fluid diapirism further gave rise to the local stress field and initiated the new fault and fractured zone,and at the same time ,many hydraulic fractures related to the overpressure fluid activity were formed.