Abstract: The dolomite in Ma-55 submember of the Lower Ordovician Majiagou formation is characterized by large thickness and rapid pinchout laterally, which is significantly different from the relatively thin dolomite with good lateral continuity in Jingbian gas field. The dolomite in Ma-55 submember in the study area is dominated by micritic dolomite, crystal powder dolomite and fine-medium dolomite, whose degree of order is relative low and proportional to crystal size. The Sr content in micrite (algae) of this horizon is far less than that of the contemporary micrite globally and the residual Sr content is affected by many factors such as fresh water and dolomitization. It is found that the higher the dolomitization degree is，the higher the contents of Fe and Mn will be. Carbon and oxygen isotope values are generally negative. The higher the dolomitization degree is，the more negative the carbon-oxygen isotope value tends to be. From the view of petrological and geochemical indices, the genesis of dolomite should be attributed to the combined action of evaporative pumping，seepage reflux and burial dolomitization