Based on the study of the felsic volcanics in Xia' nanggou district of Yidun island arc, the authors identified for the first time an intraplate volcanics belt in the back_arc region of the arc. These volcanic rocks are shoshonitic series and are geochemically characterised by K2O-, Nb-, Ta-, Zr-, Hf-enrichment and CaO-, Sr-, Eu-depletion. They have a common V-type" REE chondrite-normalized pattern. Their Pearce' s tectonic discrimination diagrams indicate a clear intraplate rifting formation setting. Four bulk-rock samples give a good Rb-Sr isochron witht=189.2±5 Ma, R=0.999 824, andISr=0.714 578. The chemical variation of the volcanics are controlled by both residual of plagioclase in the melting source area and crystal fractionation of biotite after magma formation. Considering the 213 Ma formation age of the Miange Formation bimodal volcanics in the back_arc basin east of the intraplate volcanic rock belt, it is held that the island arc completed the important transition from subduction arc-forming to intraplate rifting regime only within a period of 24 Ma, from which exact constraints on the arc evolution sequence have been obtained.