Deepwater allogene oolitic limestone can be identified by macroscopic and microscopic features, grain size characteristics and sedimentary structures. Macroscopical and microscopical observation and analysis have done to the outcrop sections and rock thin sections form Middle Ordovician Xujiajuan oolitic limestone in Zhongning Area, Ningxia. The result of statistical analysis and contrast of rock grain sizes has shown that oolites and pseudoolites, normal oolites and epidermis oolites formed in different environments. The granularity is characterized by gravity flow transportation. Combined with the sedimentary background, it is believed that this kind of oolitc limestone is deepwater allogene deposits in origin and the oolites are likely to come from the contemporaneous carbonate platform in the west margin of Ordos Block to this deep water area by gravity flow transportation.