The Sinian is extensively distributed in the Yangtze Block, South China, comprising the Doushantuo and Dengying Formations from base to top. In the recent years, hydrocarbon exploration in the Sichuan Basin has revealed that an approximately NW-SE trending intraplatform trough was formed during the Sinian, and its formation and evolution controlled the reservoir development and gas field formation of the Dengying Formation around the trough. Now the Anyue Field of the Dengying Formation with much research has been found in the eastern intraplatform trough. However, the Sinian sequence, sedimentary evolution and reservoir characteristics in the western intraplatform trough still need to be further studied. In this paper, two representative outcrops close to the western edge of the Sinian intraplatform trough, i.e. the Dashuizha and Yanziyan sections in Mianzhu County, are selected to systematically analyze the stratigraphic division, sedimentary evolution, and reservoir characteristics. The results of this paper show that: (1) The carbon isotope curves can effectively distinguish the Sinian Doushantuo and Dengying Formations. The carbon isotope at the top of the Doushantuo Formation is characterized by negative shift (up to -8‰), which is considerably compared with that at the base of the Dengying Formation with positive shift. Both the Doushantuo and Dengying Formations were developed in the western intraplatform trough. The Doushantuo Formation at least includes Members 2 to 4, and the typical cap dolomite of Member 1 is not occurred. And the Dengying Formation is completely developed, from base to top, including Members 1 to 4. (2) The Doushantuo Formation is composed of a mixed succession including the lower dominant siliciclastic rocks and the upper dominant carbonate rocks. Furthermore, the Doushantuo Formation is dominated by deep-water slope facies deposition with the features of syn-depositional slump folds, rhythmites of interbedded carbonate and mudstone, and olistostrome breccias. The Dengying Formation comprises dolomites with minor chert and mudstone, and it is mainly shallow water carbonate platform facies deposition with the development of microbial laminated dolomites and botryoidal dolomites. Thus, during the Sinian the study area experienced a sedimentary evolution from deep-water slope to shallow water carbonate platform. (3) Thick microbial laminated dolomite reservoir with large scale and high quality was developed in the Dengying Member 2 in the western intraplatform trough, which could be compared with eastern intraplatform trough, revealing that the western intraplatform trough has also the excellent reservoir conditions of large gas field formation.