Abstract: Sun Jianchu（1936） coined the term“Xidagou system”,and later Si Xingjian et al.（1962）changed the term in-to the“Xidagou Group”.For a variety of reasons ,there are no definite criteria for defining the Xidagou System or Xidagou Group.Divergent views have long existed as to its podition in Triassic continental strata in the northern Qilian area.Through two years of investgations the author has demonstrated that the present Xidatan River is pre-cisely the naming place of the original Xidatan（‘gou’ means gully in Chinese）（Sun Jianchu,1936） and measured a new section,thus laying a foundation for establishing a sequence of Triassic continental strata in the northern Qil-ian area. This paper also introduces the macroscopic features and division indicators of the lithostratigraphic units of this group.