The plagioclase crystals in trachyandesite from the Late Jurassic Manitu Formation in Wuchagou of the Da Hinggan Mountains have obvious corrosion phenomenon and well-developed zoning texture. The feldspar nomenclature diagram shows that the plagioclases consist mainly of labrador and surbodinately oligoclase and andesine. The variations of the minor elements (Fe, Sr, Ba, etc.) composition can determine the formation mechanisms of reverse zoning on the basis of investigating the causes of plagioclase zoning. According to the evidence from the electron microprobe analyses and textures of the plagioclase, the authors also analyzed the variation curves of trace elements Fe and Sr with An and, on such a basis, reached the conclusion that the zoning resulted from three magma mixing events in the process of plagioclase crystallization.