Based on observation of well core samples and sections and physical analysis for the middle array of Ordovician Majiagou dolostone reservoirs to the west of Jingbian Gas Field, it is shown that the reservoir rock mainly consist of siltcrystal dolostone, siltcrystal-micrite dolostone and micrite dolostone in lithology, which occupy 33,, 21, and 15, respectively in statistical rock sections. The types of Primary reservoir space are intercrystalline pores and intercrystalline dissolved pores in dominant, followed by karst caves and solution fractures. The reservoirs are characterized of low porosity and low permeability. The porosity is 1,~3, which takes 45.82, of total samples and the permeability is 0.001~0.1 mD which takes 74.4, . Development of the reservoirs is obviously controlled by sedimentation and diagenesis. Dolostone tidal flats and grainstone shoals are the good sedimentary facies. Dolomitization and epi-karstification are the main contribution to reservoir pore formation.