The first submember of the fifth member of the Lower Ordovician Majiagou Formation in Xingxian, Shanxi Province, consists mainly of micrite deposited in low-energy epicontinental environments and psephitic micrite resulting from reworking of original carbonates by palaeokarstification. Through leaching by meteoric water dissolving CO2 from organic carbon, the δ13C values of microcrystalline calcite range from -12.4‰ to -2.4 ‰ with a mean of -5.89‰. The unaltered marine original microcrystalline calcite has high δ18O values (-4.3‰). Most microcrystalline calcite h as low δ18O values, the lowest value being -13.1‰ and the mean being -6.85‰. It has low values of Sr and Na2O, ranging 0.13×10－６ -98×10－６ and 0%-0.04% with means of 30.5×10－６ and 0.02% respectively. The values of Fe2O3 and MnO range 0.05%-6.25% and 0.01%-0.08% with means of 1.03% and 0.02% respectively. Calcite filling has lower values of δ13C, δ18O, Fe2O3, MnO and Na2O, and a little higher values of Sr than micrite, indicating that calcite filling has been more strongly affected by leaching of meteoric water and formed at different burial depths. The homogenization temperatures of fluid inclusions of calcite filling range 52-289℃ with a mean of 162℃ and the salinity (w B)) of fluid inclusions of calcite filling ranges 0.9%-15.3% with a mean of 5.9%, indicating that calcite filling formed at relatively deep burial conditions.