Because of tunable bandgap and high carrier mobility, ternary III-V nanowires (NWs) have demonstrated enormous potential for advanced applications. However, the synthesis of large-scale and highly-crystalline In_xGa_1−xSb NWs is still a challenge. Here, we achieve high-density and crystalline stoichiometric In_xGa_1−xSb (0.09 < x < 0.28) NWs on amorphous substrates with the uniform phase-purity and -orientation via chemical vapor deposition. The as-prepared NWs show excellent electrical and optoelectronic characteristics, including the high hole mobility (i.e. 463 cm² V⁻¹ s⁻¹ for In_0.09Ga_0.91Sb NWs) as well as broadband and ultrafast photoresponse over the visible and infrared optical communication region (1550 nm). Specifically, the In_0.28Ga_0.72Sb NW device yields efficient rise and decay times down to 38 and 53 μs, respectively, along with the responsivity of 6000 A W⁻¹ and external quantum efficiency of 4.8 × 10⁶ % towards 1550 nm regime. High-performance NW parallel-arrayed devices can also be fabricated to illustrate their large-scale device integrability for next-generation, ultrafast, high-responsivity and broadband photodetectors.