Abundant mixed carbonate—siliciclastic sediments and mixed sedimentary successions develop in the Third Series of Cambrian in Hunan Province. And their sedimentary characteristics, spatial and temporal distribution and origin are significant for construction of the sedimentary environment and evolution of carbonate platform in the southeastern margin of the Middle—Upper Yangtze region.Methods:Field observation and petrologic analysis are used to investigate sedimentary characteristics of outcrops and grain/mineral compositions of the mixed carbonate—siliciclastic sediments. On the basis, types of lithological association and sedimentary characteristics of the mixed sedimentary successions and mixed carbonate—siliciclastic sediments were summarized for the first time. Results:There are 4 types of mixed sedimentary successions with 6 kinds of lithofacies zones developing in Third Series of Cambrian of Hunan, to the north of the Jingzhou—Lianyuan—Longhui—Ningxiang areas. Moreover, the mixed sedimentary successions occur in the northwestern and central Hunan Province. Different types of the mixed sedimentary successions present different sedimentary characteristics. It is inferred that they were possibly produced in the shallow water mixed shelf influenced by periodic storm and the deep water mixed shelf with low hydrodynamic conditions. The mechanical transport including gradual facies change and mutational deposited by event was the main origin of the mixed carbonate—siliciclastic sediments. The construction of the mixed shelf indicates that there was no platform margin in the southeastern margin of the Middle—Upper Yangtze region during the Third Period of the Cambrian. Therefore, there was not the rimmed carbonate platform, and the mixed carbonate—siliciclastic sediments of this study should be a relatively gentle carbonate dominated neritic mixed shelf with some amounts of terrigenous clastic input