It is demonstrated that the Middle and Upper Cambrian reservoirs in southern part of Sichuan Basin are sediments in a restricted carbonate platform, which are mainly composed of silty-fine dolostone,dolorudite, dolorenite, oolitic dolostone, and so on. Diagenensis mainly include compaction, cementation, dolomitization, dissolution and fracturing. Dolomitization and karstification are the keys of forming excellent reservoirs. The dolostone that occurred in bank environment in this area is originated from mixed dolomitization. Karstification that took place during several phases includes depositional, emerged, buried and folded karst, in which the emergence karst is the main karstification. Compaction and cementation destroyed a majority of primary and secondary pores, which resulted in decreasing porosity to 2,～3, from about 15,. Consequential karstification and fracturing brought on increasing porosity to 4,-5, in parts of the reservoirs　and intracrystalline pores and intracrystalline solution pores are common.