Several layers of oncolitic limestone that are subordinate to Middle Ordovician Majiagou Submember-55 are discovered at Guanjiaya outcrop in Xingxian area, Ordos Basin. Based on the outcrop observation and microscopic thin-section identification to lithologic characteristics of the oncolitic limestone, some experimental analysis of outcrop samples were carried out, including whole rock X-ray diffraction, major and trace elements, C, O and Sr stable isotopes and organic carbon. It is indicated that the oncolitic limestone formed in tidal flat environment in open sea, and recrystal ization of oncolite took place in the penecontemporaneous stage. Megacrystal ine calcites presented mosaic contact under compaction and pressuring twin-crystal lamellae formed in shallow burial stage. The oncolitic limestone at the bot om of Submember-55, which formed during sea level rising, was not affected by early freshwater leaching and corrosion. Otherwise, the oncolitic limestone on the uppermost, which formed during sea level fal ing, was evidently affected by the leaching and corrosion. Led by underground hydrotherm, isotope exchange, organic maturation and hydrocarbon expulsion took place in water-rock interaction during the moderate-deep burial stage. The discovery of this set of oncolitic limestone makes it clear that the Majiagou Submember-55 limestone formed in the environment of open sea, which can provide important reference for the research of Ordovician Majiagou source rock.