One-dimensional (1D) heteroepitaxy with an abrupt interface is essential to construct the 1D heterojunctions required for photonic and electronic devices. During catalytic 1D heteroepitaxial growth, however, the heterojunctions are generically kinked and composition-diffused across the interfaces. Here, we report a simple synthetic route for straight 1D heteroepitaxy with atomically sharp interfaces of group IV(Ge)/group II-VI(ZnSe) nanowires (NWs) during Au-catalytic growth. Specifically, it is discovered that eliminating residues in Au catalysts by Se vapour treatments lowers the energy barrier for the Ge NW axial heteroepitaxy on ZnSe NWs, and forms atomically abrupt heterointerfaces. We verified such 1D variation in the local electronic band structure of the grown Ge/ZnSe NW heterojunctions with spatially resolved photocurrent measurements.