Abstract: Similar to the δ¹³C_PDB values for the cap carbonates overlying the Marinoan glacial diamictites in the other parts of the world, there is a markedly negative excursion of the δ¹³C_PDB values for the Neoproterozoic cap dolostones in northwestern Fanjing Mountains, Guizhou. The δ¹³C_PDB values range from -1.4‰ to -8.52‰, with an average value of -4.52‰, and the δ¹⁸O_PDB values range from -5.44‰ to -11.34‰, with an average value of -8.68‰. Except two data for the cap dolostones at the top, the rest δ¹³C_PDB and δ¹⁸O_PDB values generally increase upsection, and have a slightly positive correlation. The examination of sedimentary structures and carbon isotope shows that the cap dolostones in the study area are not related with the release of solid methane from the submarine permafrost soil. Instead they should be the products originated from the rapid precipitation of the weathered terrigenous alkaline matter following the reaction with CO₂ due to the warmer climates after the Nantuoan glacial stage. This explanation strongly supports the "snowball Earth" hypothesis.