Abstract: Well Xike-1 is the most complete coring well to reveal the reef formation in the South China Sea. Tight dolostone is developed in the Miocene Sanya Formation, with porosity of 1.65% ～16.4%, and an average of 6.7%. The local permeability is only (11.8～39)×10⁻³ μm², and the average permeability is less than 1×10⁻³ μm². Based on sedimentary petrology, petrophysical properties of porosity and permeability and inclusion testing methods, the diagenetic characteristics of tight dolostone are analyzed, and the diagenetic evolution conditions are discussed. The dolomite in the tight dolostone is mainly composed of powder crystal, fine crystal and mud crystal. The morphology of dolomite is mostly euhedral and subhedral. The crystal shape is good, with fog center and bright edge structure. The dolomite crystal contacts closely. The intergranular pores are fine, separated and disconnected, and the secondary corrosion pores are not developed. The tight dolostone of Sanya Formation was formed in the stage of syngenetic and post genetic rocks. Frequent sea-level changes and sediment exposure lead to multi-stage seawater filling porosities, coupled with high temperature and high salinity paleo marine and stratigraphic environment, which create conditions for the formation of tight dolostone.