The measured profile rock of Middle Triassic Longtou Formation that is cropped out at Qingyan in the south part of Guiyang, Guizhou is the shoal sediments of platform margin. The Longtou reservoir mainly consists of bioclastic limestone oncolitic limestone, algal limestone and micritic limestone. By analysis of cast thin sections and SEM, it is identified that micropores, intercrystalline pores, intragranular dissolved pores, molded pores and structural microfissures. Analysis of reservoir physical property for 44 field samples has shown that only 4.5, of samples can be sorted well with good reservoirs. it is deemed that sedimentation is of a certain control to the reservoir. Cementation, recrystallization made the whole Longtou reservoir poor physical property but the dissolution, dolomitization and tectonism partly improve the reservoir. It is indicated that the fracture zone in algae subfacies is the favorable area of reservoir development with hydrocarbon potential.