This paper studied the petrologic feature, diagenetic characteristics, the sequence of diagenesis and diagenetic stage division of Es4 reservoir in different structural belt of Langgu Sag. The results show that the reservoirs experience compaction, cementation, metasomatism, dissolution and matix recrystallition. The diagenesises in Liuquan-Caojiawu are strong dissolution of feldspar and debris, in Jiuzhou-Guan structural belt, they are compaction, matix recrystallition and a little carbonate cementation. The sequences of diagenesis are different in each structural belt. In study area, the Es₄ reservoirs above 2 640 m have been in stage B of early diagenetic stage, 2 640~3 400 m in stage A1 of middle diagenetic stage, 3 400~4 250 m in stage A2 of middle diagenetic stage and under 4 250 m in stage B of middle diagenetic stage.