The internal photoemission process of hot electrons generated by light excitation in metals can break the bandgap limitation of semiconductors and it is of great significance in the applications of photodetections， photovoltaics， and photocatalyses. However， the effects of the morphology， size， and Schottky junction barrier of metallic nanostructures on the injection efficiency of hot electrons are still unclear. Here， based on Monte Carlo simulations， the carrier transport behaviors in single- and double-junction planar structures and single- and double-junction core-shell nanowires are studied， and the effects of the morphology， size， and Schottky junction barrier on the injection efficiency of hot electrons are analyzed. The results show that in the planar hot electron device with a thick metal film， the double Schottky barrier configuration can reduce the hot electron thermalization loss and significantly increase the injection efficiency； with the thin metallic film， the thermalization loss of hot electrons is small， and the difference in injection efficiency between the double- and single-junction configurations is negligible. In the single-junction core-shell nanowires， the electron-phonon scattering significantly improves the hot electron injection efficiency， which decreases （increases） with the increasing metal thickness （nanowire radius）. In the double-junction core-shell nanowires， the injection angles of hot electrons at the outer Schottky junction interface are smaller than that of the inner junction interface， so the outer Schottky junction has a stronger hot electron harvesting capability. As the metal thickness （nanowire radius） increases， the number of hot electrons generated near the outer （inner） Schottky junction increases， resulting in an increase （decrease） in injection efficiency. This study provides an in-depth understanding of the hot electron transport behavior and the injection efficiency limit in various metallic nanostructures， and provides a guidance for the design of high-performance hot-electron devices.