Based on the cores, thin sections, well logging data, and geochemical parameters of pore filling, the characteristics and main controlling factors of weathering crust reservoir in Lower Ordovician Majiagou Submember-51-4, Yichuan-Huanglong area of Ordos Basin were studied. It′s shown that the primary reservoir is dolostone while the secondary reservoir is limestone. The reservoir space is mainly composed of anhydrite moldic pores, and secondly composed of dissolution pores and intergranular pores. The fractures have a certain effect on the reservoir permeability. In general, the reservoirs are of low porosity(mainly less than 2,) and low permeability (less than 0.01×10-3μm2). Controlled by sedimentation and diagenesis, the grain-beach and anhydrite concretionbearing dolomite flat are the favorable facies, and karstification and dolomitization are the most important constructive, while associated infilling, compaction, and dedolomitization are mainly destructive.