Mixed sediments of carbonate and clastic origin are widely developed in Shiniulan formation of lower Silurian, southeast of Chongqing. Taking the typical Shiniulan profile as the study object, through the field observations, thin section analysis, isotope analysis and other means, the mixed sediments and the constraints on the reef of Siniulan formation are studied. The results show that, there mainly developed argillaceous limestone and sandy mudstone, micritic limestone and mudstone, biological limestone and lime mudstone, which have formed three diamictic strata. There develops the argillaceous limestone, sandy limestone, silty clay limestone, silty reef limestone (biogenic limestone), gray siltstone, lime mudstone, that is, six kinds of gray diamictites. In Siniulan formation, the depositional environments are mainly early shallow mixed continental shelf, that is, interim reef deposits-advanced reef slope sedimentary evolution, and the terrigeneous content reduces from bottom to top in the overall process, and the mixed deposition occurs mainly in shallow mixed continental shelf and reef and beach/ramp, two depositional environments. It is confirmed that, in the Shiniulan reef development process, the injected terrigeneous debris is the major cause which has constrained the development of the reef and the biological communities.