Drowned unconformity sequence is marked by a sequence interface on which a condensed section(CS) directly covers, which means a “CS+HST” type. It is distinguished with Type-Ⅰ and Type-Ⅱ sequence interfaces defined by sequence stratigraphy due to no evidence of exposure so that it becomes a special third-order carbonate cyclic sequence interface. Through the description of features, the introduction of identification methods and the analysis of examples, it is explained that the drowned unconformity sequence is a special type of stratigraphic cyclic sequence, that is the "deepened submerge sequence interface", which plays an important action in hydrocarbon migration.