Since the discovery of high quality dolomite reservoirs in Submember Ma55 of the Ordovician Majiagou Formation (O1m55) in Daniudi area of the Ordos Basin, the dolomite genesis has become the key to natural gas exploration in this interval. Based on petrological characteristics, the geochemical characteristics and genesis of O1m55 dolomite in Daniudi area are systematically studied with geochemical test data such as isotopic compositions, trace element contents, and fluid inclusion data. The results show that: (1) There are two types of dolomite, i.e., micrite dolomite and very fine-fine crystalline dolomite in O1m55, and the former mainly developed at the bottom, the latter mainly developed in the upper-middle part. (2) In the micrite dolomite, the original sedimentary structures such as horizontal algae lamina, bird-eye and acicular gypsum pseudocrystal are developed, the pores were not developed. The order degree of dolomite was low, with an average value of 0.63. The average value of δ13C and δ18O are -0.65‰ and -6.65‰ respectively. The content of Sr and Na (151 μg/ g and 488 μg/ g respectively）is high, and the content of Fe and Mn (1 202 μg/ g and 49 μg/ g respectively) is low. The salinity (24.2，) during the formation period is basically consistent with that of micrite limestone. It is inferred that micrite dolomite should be the product of the quasi syngenetic stage in tidal flat environment of limited platform. (3) In the very fine-fine crystalline dolomite, the sedimentary structures of residual sand debris and "fog core and bright edge" are common, and the intercrystalline pores are developed. The order degree of dolomite is relatively high, with an average value of 0.89. The average values of δ13C and δ18O are -1.33‰ and -8.91‰ respectively. Compared with the micrite dolomite, the content of Sr and Na（78 μg/g and 426 μg/g respectively）is low, while the content of Fe and Mn ( 1 974 μg/g and 77 μg/g respectively) is high. The salinity (22.6，) during the formation period is similar to that of micrite limestone. It is inferred that the very fine-fine crystallinefine dolomite should be the product of reflux infiltration in shallow burial stage in tidal flat shoal environment. (4) The dolomitization fluid of O1m55 is mainly sea-sourced, without the participation of atmospheric fresh water and hydrothermal fluid. Based on the above petrological and geochemical characteristics, the quasi syngenetic and reflux infiltration dolomitization models of the O1m55 in Daniudi area are established, which provides a theoretical guidance for the prediction of high quality dolomite reservoirs and the selection of geological exploration directions in the study area.