Abstract: In order to know the formation surroundings of the Ordovician dolostone that is Lower Paleozoic reservoir of natural gas in the Ordos Basin,the relationship between its genetic type and carbon and oxygen isotope is studied.Carbon and oxygen isotope of penecontemporaneous dolostone is heavy,whose δ¹³C value varies from 0.639‰ to -2.2‰ and δ¹⁸O value does from -0.2‰ to -8.668‰.The penecontemporaneous dolomite rock forms in saline water body.δ¹³C value of diagenetic dolostone varies from 0.9‰ to -3.2‰ and δ¹⁸O does from -1.7‰ to -7.067‰.The diagenetic dolostone froms in high-magnesian saline water body.Carbon and oxygen isotope of immerged dolostone is light,whose δ¹³C value varies from 0.580‰ to -3.7‰ and δ¹⁸O value does from -6.88‰ to -9.97‰.The formation surrounding of immerged dolostone is higher temperature,lower salinity and lower-magnesium.δ¹³C value of carbonate vein in dolostone most is from -1.00‰ to -5.66‰,while that in limestone most is from -1.30‰ to -8.65‰.Oxygen isotope of carbonate vein is lighter with narrow ranges,whose value varies from -10.66‰ to -18.91‰,and its Z value most is 100 and 110.Those indicate there was fresh-water leaching during formation of carbonate vein.