The Upper Permian Changxing reef limestones deeply buried in the platform margin by the southwe st side of Kaijiang-Liangping Trough in the east of Sichuan Basin. These limestones are commonly tight due to paucity of large-scale dolomitization though they possessed high depositional porosity. It is shown that the paucity of large-scale dolomitization accounts for dolomitization fluid in short supply during early diagenesis rather than what was supposed by previous studies that dolomitization fluid could not enter the pores blocked by early cements. In the light of petrography, geochemistry and fluid-inclusion analysis, it is revealed that the facies-selective dolomitization in Changxing reef limestone implies its relation with sedimentary environment. The reason why pores are hardly preserved in the reef limestones may owe to the cementation of the radiaxial calcite precipitated from normal sea water during early diagenetic environment and the blocky sparry calcite formed during deep-buried diagenetic environment though a great amount of pores and space of fluid activity were stil present within reef limestone at the end of early marine cementation. Because the Changxing reef limestones (reef cores) are just located in high energy zone in the platform margin nearby the trough, the diagenetic fluid is actively circulating normal seawater under the control of sedimentary environment, where seepage reflux of mildly-evaporating seawater was not provided for dolomitization.