Mesozoic magmatic activities occurred frequently in northeast of China. The Xintun high-Mg diorites in the Yanbian area are a special type of rock, study of which is of great significance to constrain its tectonic background. The Xintun diorites are characterized by high Mg values (47~64), enrichment of LILEs and LREEs, and depletion of HFSEs and HREEs, with their εHf(128 Ma) ratios of 3.2~8.9，indicating that the mantle sourced from the depleted mantle source. The higher La/Sm ratios and Th/Yb ratios, and lower Ba/La ratios indicate the contribution of subduction sediments to the Xintun diorites. Petrologic features show that early crystallization of hornblende, magnetite and other minerals indicates that the initial magma was a system of rich water and high oxygen fugacity. Under the condition of water saturation, the crystallization of hornblende, biotite, magnetite and other minerals prior to plagioclase resulted in increasing of Sr/Y ratios and fractionation of LREEs and HREEs. During the period of 130~110 Ma, rollback of PaleoPacific slab resulted in backarc spreading of the northeastern region and upwelling of asthenosphere materials changed the heat state of mantle wedge, finally generating partial melting of subductd sediments and forming early Cretaceous highMg diorite through metasomatizing overlying mantle wedge.