The Yixian Formation is a series of volcanic-sedimentary rocks in Biepiao area of Liaoning Province． It is mainly composed of basic and intermediate-basic volcanic lava，pyroclasts and terrestrial sedimentary rocks． Based on the regularity of volcanic activity，the Yixian Formation was divided by the present authors into four members in ascending order: the first member is of basal conglomerate，basic and intermediate-basic volcanic rocks; the second member is of lake phrase sedimentary rocks，or in another word，precious fossil-rich sedimentary beds; the third is of basic volcanic rocks; and the fourth is of upper conglomerate． Field mapping and comprehensive study also indicate that there are abundant vertebrate fossils ( mainly of Psittacosaurus) in the first member of the Yixian Formation，and the Jehol Biota ( including Sinosauropterxy，Confuciusorns sanctus， Archaefructus，etc． ) is yielded in the second member of Yixian Formation． From west to east，the volcanic activity of Yixian Formation changed regularly from early to late，and from basic and intermediate-basic to acid ( alkali) ．