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We present high‐resolution 3D images of the crustal S‐wave velocity structure beneath northeastern Tibet, via ambient noise imaging techniques. The data come from continuous recordings taken in one year by a seismic array consisting of 366 broadband stations in the study region. The results show a substantial and pervasive middle crust low‐velocity structure (<3.4 km/s) beneath the plateau region, which partially protrudes northeastward across the West Qinling fault, reflecting channel flow from inside the plateau and material extrusion in the periphery. The growth of the northeastern Tibetan Plateau in the study region is manifested in its northeastward expansion and thickening driven by crustal channel flow, accompanied with upper crust shortening. Plain Language Summary: The growth mechanisms of the Tibetan Plateau (TP) have been considered controversial for decades. Expansion of the northeastern TP and the interaction with its peripheral area have become of major interest to researchers. In this study, we present a high‐resolution 3D image of the crustal S‐wave velocity structure beneath this area by analyzing the continuous ambient noise data recoded by a dense seismic array via ambient noise imaging techniques. The results show a substantial and pervasive middle crust low‐velocity structure beneath the plateau region, which partially protrudes northeastward across the West Qinling fault, reflecting channel flow from inside the plateau and material extrusion in the periphery. The channel flow slows down gradually with its outward protrusion. The upper and lower crust is decoupled. In addition to the crustal channel flow, upper crust shortening takes part in the growth of the northeastern Tibet. Key Points: High‐resolution 3D crustal shear‐wave velocity images beneath the northeastern Tibetan margin via ambient noise imaging techniquesSubstantial and pervasive middle crustal channel flow is found beneath the northeastern TibetThe northeastward expansion of the northeastern Tibet is driven by crustal channel flow, accompanied with upper crust shortening [ABSTRACT FROM AUTHOR]