Nanostructured metal phosphides are very attractive materials in energy storage and conversion, but their applications are severely limited by complicated preparation steps, harsh conditions and large excess of highly toxic phosphorus source. Here we develop a highly efficient one-step method to synthesize Sn₄P₃ nanostructure based on simultaneous reduction of SnCl₄ and PCl₃ on mechanically activated Na surface and in situ phosphorization. The low-toxic PCl₃ displays a very high phosphorizing efficiency (100%). Furthermore, this simple method is powerful to control phosphide size. Ultrafine Sn₄P₃ nanocrystals (< 5 nm) supported on carbon sheets (Sn₄P₃/C) are obtained, which is due to the unique bottom-up surface-limited reaction. As the anode material for sodium/lithium ion batteries (SIBs/LIBs), the Sn₄P₃/C shows profound sodiation/lithiation extents, good phase-conversion reversibility, excellent rate performance and long cycling stability, retaining high capacities of 420 mAh/g for SIBs and 760 mAh/g for LIBs even after 400 cycles at 1.0 A/g. Combining simple and efficient preparation, low-toxic and high-efficiency phosphorus source and good control of nanosize, this method is very promising for low-cost and scalable preparation of high-performance Sn₄P₃ anode.