Abstract: The essential sequence boundary is a major transformation in the vertical plane. It can be divided in downward succession into three groups, i.e., cycle/non-cycle essential sequence boundary (level Ⅰ interface), cycle/rhythmic essential sequence boundary (level Ⅱ interface) and fining-upward graded/coarsening-upward graded essential sequence (level Ⅲ interface). These interfaces are widely developed in clastic sedimentary rocks, and hence are important boundaries for subdividing geological mapping unit in detail. This unit is an important complement for traditional layering of sedimentary sequence and interface choosing for geological mapping unit. More importantly, it is a guide to the division of geological mapping unit, the analysis of sedimentary facies and the study of regional geology for particularly thick clastic units in the orogenic belt.