Abstract: Six years after the discovery of tight dolomite and ferroan dolomite in Shidao island, further studies have focused on the petrographic characteristics. The latest sampling of the tight dolomite strata in well Xike 1 was completed, and the systematic routine rock slice grinding and polarizing microscope and scanning electron microscope identification were carried out, which provided a series of photos and understanding of identification analysis. The petrographic microscopic characteristics of deep dolomite under polarized light microscope were determined, and the characteristics of tight algal reef dolomite composed of microcrystallite-fine-crystallite-clotted micritic dolomite were revealed. They developed fine crystallite-fine-grained, clotted and biological structure. Fine grained dolomite idiomorphic to hypoidiomorphic, bright edge of fog center, 0.05～0.25 mm, multi-generation characteristics; The bioclasts are coralophyta and cellophyta dolomite, 2～3 mm, some 3～5 mm, a few 5～10 mm. The internal dolomite of the organism is mostly silt-crystal, growing or filling in the bioclastic pores. The dolomite in the algae is irregular, 0.3～0.5 mm, scattered and inlaid, involving opaque minerals. Massive and irregular clumps of micritic dolomite were found, and some clumps gathered together. The inside of the clumps was mainly filled with micritic dolomite and a small amount of organic matter, and cryptocrystalline structure was developed.Multiple depth dolomite content of 90%～95% or 95%～100%, under the mirror colorless, sometimes cloudy gray, flash prominent, complete cleavage, senior white interference color; Some of them contain 5%～10% fine crystalline calcite, semi-idiomorphic, 0.05～0.25 mm, scattered among dolomites. The silt-crystal calcite is idiomorphic to hemiidiomorphic granular, 0.01～0.05 mm, Mosaic distributed in the edge of the bioclasts or filled in the internal pores of the bioclasts; The scanning electron microscope (SEM) system analysis is mainly carried out on the tight dolomite samples, and the typical photos are selected to analyze the dolomite grain and filling characteristics. It is found that there are two kinds of filling styles, one is micrite filling outcrops in the pores and cracks between the fine-grained dolomite, the other is micrite area coverage on the fine-grained dolomite.