Abstract: Located on the south margin of the Xing'an-Mongolian orogenic belt, Bainaimiao arc is an important part for studying the early Paleozoic geology of the Paleo-Asian Ocean, but its basement property is still controversial.During the geological survey in Ar Horqin Banner, eastern Inner Mongolia, the authors discovered a set of Paleoproterozoic metamorphic complexes in Shuangsheng area, which provides new evidence for the study of the basement property of Bainaimiao arc.In this paper, amphibolite in the metamorphic complex was studied in the aspects of petrography, geochronology and geochemistry.The results show that the protolith of the amphibolite belongs to gabbronite, and had undergone at least two stages of metamorphism at 1924±17 Ma和1814.2±9.4 Ma respectively.Geochemical features show that the protolith of the amphibolite belongs to tholeiitic series, characterized by relatively high Fe, Mg, Ca, Ti, and Na₂O/K₂O(2.05~3.12), medium Mg^#(41.09~51.71), and relatively low alkali, with curve characteristics of rare earth and trace elements similar to those of OIB, suggesting that the protolith originated from lithosphere mantle and was formed in ocean intraplate.The metamorphic complex from Shuangsheng area may have recorded the event of ocean-continent transformation during the assembly of the Columbia supercontinent in the Paleoproterozoic.Combined with regional data, it is concluded that Bainaimiao arc had a uniform Precambrian basement, which had tectonic affinity to the North China Craton definitely.