Abstract: There is a suite of middle- high grade metamorphic rocks in Boin Sum region, Inner Mongolia which was named as Baiyinduxi Group. The rock assemblage of the group consists of leptynite, mica shist and amphibolite. There are different opnuons in tectonic interpretation among the geologists, who work over there. A representative one considers that the Baiyinduxi Group was the product of island arc or other active continental margin. Based on the field and experimental works, we have gotten some new idea. The original rocks of leptynite are considered to be feldspar-quartz sandstone and mudstone. The important characteristics of these rocks are low maturity in fabric, high maturity in composition and well- developed rhythm. It implies that the original rocks would be formed in an instable basin on the continental crust. The original rocks of amphibolite in Baiyinduxi Group are considered to be basic volcanic rocks. The chemical composition of amphiblitic rocks is：SiO2, 49.93:Ca , 8.91:MgO, 4.24:Na20+K20, 4.69 in average and FeO>Fe2O3:Na20> K2O, LREE/HREE =4.8. In chemical composition they are similar to continental tholeiite. These basic volcanic rocks have εNd(t)= -1.1～+1.4. It means that the magmas of basic volcanic rocks come from mantle and are mixed with continental materials. However, the contents of trace elements are higher than those of volcanic rocks in active magin. Its tectonic environment would be continental margin. According to the characteristics of the rock assemblage, like relatively large amount of quartz fragment, low maturity in fabric, high maturity in composition, well- developed rhythm and other chemical characteristics, Baiyinduxi Group would precipitate in all environment of volcanic passive continental margin. It indicates that the tectonic environment in Boin Sum region was extentional transitional type of continental crust in Middle- Neo Proterozoic rather than an island arc assemblage that was recognized before Rifting was the most important event of North China Platform Based on the new data described in this paper, it may be suggested that the north margin of North China Plate underwent a large scale rifting or down- warping in Middle Neo Proterozoic and a volcanic passive margin was formed. Baiyinduxi Group might be the product of that event. At the end of Neo-Proterozoic, the violent tectonic movement ended the evolution history of passive continental margin and began an active margin regime.