In order to study the characteristics and genesis of dolomite in the main gas reservoirs of the Lower Paleozonic submember Ma55 in the Sudong region，methods such as thin section observation，X-ray diffraction，scanning electron microscopy，cathodoluminescence and trace elements analysis were used to analyze the dolomite samples. The results show that the Lower Paleozoic submember Ma55 in the Sudong region are characterized by dolomitization，which can be concluded as follows：There are three types of dolomite：penecontemporary dolomite，reflux seepage dolomite and buried dolomite in submember Ma55 of the Majiagou Formation. The original intergranular pores developed well in the dolomite reservoirs，and they are dominated by intercrystal pores and suintercrystal dissolved pores. Occasionally，some non-constitutive disso? lution pores，residual intergranular pores and a small amount of paste pores were observed. Cathodoluminescence experimental results show the presence of light-emitting micrite dolomite（containing sand-gravel clastics）indicating the development of muddy gravel dolomite in the study area. The X-ray diffraction results show that the order degree of penecontemporary dolomite is not high，average 0.443，and reflux seepage dolomite has a high degree of order，average 0.574. The ordered degree of buried dolomite is as high as 1. The dolomite composition is simple，and the ratio of Ca2+ and Mg2+ is close to the stoichiometric CaMg（CO3）2，which verifies the accuracy of the order degree of dolomite obtained by X-ray diffraction.And the main trace element characteristics indicate that the reflow infiltration dolomite mainly occurs in seawater diagenetic medium.