The impact toughness anisotropy of Ti80 alloy bars and its internal causes were systematically studied in this paper. Charpy impact test results show that the impact toughness of C-R specimen (the surface of the notch is perpendicular to the axial direction of the bar) at R/2 position is much higher than that of C-L specimen (the surface of the notch is parallel to the axial direction of the bar). The metallographic analysis shows that the transverse microstructure of the heat-treated sample is mainly composed of uniform equiaxed primary α phases, and the microstructure at the corresponding longitudinal streamline shows a long strip primary α phase. The results of impact fracture Scanning Electron Microscopy (SEM) indicate that the cross-section of C-R specimen undulates more violently, there are a large number of transverse cracks at the cross-section, the crack growth direction is perpendicular to the direction of the primary α phase being elongated, and the long strip primary α phase plays a key role in hindering crack propagation, thus showing significantly higher impact toughness than that of C-L specimens. In addition, The results of Electron Back Scatter Diffraction (EBSD) on the side of the fracture (perpendicular to the surface of the notch) show that the C-R specimen experienced more severe plastic deformation during the initiation and propagation of cracks, a process that consumes more energy and thus has higher impact toughness.