It is shown that silt-sized crystalline dolostone with moldic-pores are common in the Ordovician weathering karst crust reservoir in the eastern part of Ordos Basin. The pore space is mainly dissolved in the reservoir and the quality of reservoir is controlled by the filling degree and pore style. Calcite filling may have some destruction but have some construction to reservoir space due to relatively large crystal grains of filling calcite with intercrystal porosity. The analysis result of weathering karst crust reservoir and pore fills has shown that the filling calcite deposited during the hyperdiagenesis expose phase while the dolomite is the product of compaction release water karst during burial time. It is put forward that the key controlling factor of favorable reservoir lie on the compaction-release water karst during middle- to deep-burial phrase.