Abstract: The Miocene reef deposits penetrated by the Well Xichen-1 have been treated as a kind of algal reef dolostone for long. This article is a summary of the data from three drilling holes on three islands of two atolls of middle Miocene-late Miocene. Comparative analysis suggests that the basic characteristics of the algal reef dolostone are rather similar each other even small differences occur in their buried depth and thickness. Most of the deposits, according to the data from drilling holes, are algal reefs and their derives, which are of significance to the understanding of the Neogene evolutionary history of the Xisha Islands. There are three dolomite layers, with a total thickness of 724.65 m. 381 pieces of samples have been taken for conventional laboratory study of thin sections, scanning electronic microscopy (sem), X-ray powder diffraction and physico-property test and analysis. Priority is given to the algal reef dolostone itself. They are buried in different depths and consist of dolomite mud with various primary pores, such as intergranular pore and visceral foramen, and secondary pores, such as moldic pores, intergranular emposieu and tectonic fissures. High porosity and permeability layers may be formed by the combination of primary and secondary pores to form favorable oil and gas reservoir. The dolomite is low in strontium content, of which the origin is related to the mixing of fresh water and sea water during the glacial regression.