Abstract: In the Upper-Yangtze region, especially in the Guizhou Province and its adjacent areas, the Middle Cambrian is well developed and is marked by a succession from small-amount limestones to large-amount dolomites. In the shallow intra-platform setting of the northwestern part of the study area, the middle Cambrian is made up by both the thin muddy and sandy dolomites with trilobite fossils of the Douposi Formation and the thick dolomites lacking fossils of the Loushanguan Group. In the deep-water setting of the southeastern part of the study area, the Middle Cambrian includes the Duliujiang Formation and the lower part of the Yangjiawan Formation that are constituted by black shales, muddy shales, muddy limestones and breccia limestones. In the transitional region, the middle Cambrian can be divided into Kaili and Jialao Formations marked by muddy shales and muddy limestones. Both the regularly temporal succession of sediments and the spatially distributional pattern of sedimentary facies form a particular sequence-stratigraphic framework that contains three third-order sequences. This sequence-stratigraphic framework not only demonstrates two types of facies-changing surfaces and two types of diachronisms in the stratigraphic records but also means a regular deepening-shoaling process of sedimentary environment controlled by cycles of the third-order sea-level changes. Series of panel diagrams of sequence-stratigraphic framework and corresponding maps of lithofacies and palaeogeography can overally illustrate the temporal evolution succession of sediments and spatial distributional pattern of sedimentary facies for the third-order sequences, and can demonstrate the forming palaeogeographic setting for the third-order sequences. More interestingly, the large-scale transgression after a large-scale regression at the turn from the early to the middle Cambrian apparently resulted in expansion of the ecological space. With this ecological expanding two famous biota＇s that are named as ＂the Taijiang Biota＂ in the end of the early Cambrian and ＂the Kaili Biota＂ of the initial stage of the middle Cambrian are formed in the deeper background of sedimentary environment. These two famous biota＇s represent another biological explosion after the biological explosion in the early stage of the early Cambrian represented by both ＂the Small-Shelly Biota＂ of the initial stage of the early Cambrian and ＂the Chengjiang Biota＂ of the early of the Early Cambrian. Therefore, together with the genetic relationship between the mass-extinction event the genetic relationship between the biodiversity event and the transgression become an important problem that needs to be studied in the future