High-rate and ultralong-life performance is one of the main objectives for the fast development of rechargeable lithium-ion batteries. Here we present a highly efficient design of off-stoichiometric Li3-3x V2+x (PO4)3/C (x = 0–0.12) nanocomposites as cathode material. The nanocomposites are crystallized along with one-step method by in-situ carbonization. Among them, Li3-3x V2+x (PO4)3/C nanocomposites with x = 0.10 exhibit the most excellent performance. The initial discharge capacity is 131 mAh g−1 (theoretical capacity is 133 mAh g−1) when cycled at a rate of 0.5C in the range of 3.0–4.3 V, and 121.6 mAh g−1 at 10C. Even at 20C, it still delivers an initial discharge capacity of 92.5 mAh g−1, and a remarkable capacity of 85.1 mAh g−1 after 1000 cycles, corresponding to capacity retention of 92%. The excellent performance is attributed to the improved lithium ion mobility and electronic conductivity. Our work provides an efficient technique for the large-scale industrial production of Li3V2(PO4)3 for ultrafast rechargeable lithium-ion batteries.