Surface passivation of semiconductor nanowires (NWs) is important for their optoelectronic properties and applications. Here, the in situ passivation effect of an epitaxial InP shell and the corresponding photodetector performance is experimentally studied. Compared with the unpassivated GaAs1- x Sb x core-only NWs, the GaAs1- x Sb x /InP core/shell NWs have shown much stronger photoluminescence and cathodoluminescence intensities. Correspondingly, the fabricated single GaAs1- x Sb x /InP core/shell NW photodetector shows a responsivity of 325.1 A W-1 (@ 1.3 μm and 1.5 V) that is significantly enhanced compared to that of single GaAs1- x Sb x core-only NW photodetectors (143.5 A W-1), with a comparable detectivity of 4.7 × 1010 and 5.3 × 1010 cm√Hz/W, respectively. This is ascribed to the enhanced carrier mobility and carrier concentration by the in situ passivation, which lead to both higher photoconductivity and dark-conductivity. Our results show that in situ passivation is an effective approach for performance enhancement of GaAs1-x Sb x NW based optoelectronic devices.