The widely distributed carbonate reservoirs of lower to middle Ordovician are the most important petroleum reservoir in Tarim Basin. Based on the geological setting and lithology, the analysis of paleoenvironment was performed. The results show that carbon isotope composition and evolution are consistent with the variations of sedimentary environments. Low delta C-13(-2.6 parts per thousand to -1.7 parts per thousand) values in the upper part of Yingshan Formation correspond to restrict platform with a low value of delta Ce. Higher delta C-13 values (-1.5 parts per thousand to 0.8 parts per thousand) from the Penglaiba Formation to lower Yingshan Formation and Lianglitage Formation correspond to open platform .