Developing advanced electrode materials and understanding their reaction mechanisms are two crucial issues for development of high-performance sodium ion batteries (SIBs). Herein, we synthesized a bimetallic single-phase nanoporous (NP) SnSb alloy with a bicontinuous ligament-channel structure through elaborate design of ternary Mg-Sn-Sb precursor and chemical dealloying. As an anode for SIBs, the NP-SnSb alloy delivers high specific capacity, excellent cycling stability (506.6 mAh g−1 after 100 cycles at 0.2 A g−1; 457.9 mAh g−1 after 150 cycles at 1.0 A g−1) and superior rate capability (458.2 mAh g−1 at 10 A g−1). Moreover, the Na3V2(PO4)3 (cathode)/NP-SnSb (anode) full cell also exhibits outstanding electrochemical performance (cycling stability and rate capability). The unique nanoporous structure (with ligaments of 38.9 ± 7.3 nm), the alloying effect and the synergetic reaction of Sn/Sb account for the eminent electrochemical properties of NP-SnSb. Most importantly, the Na storage mechanism of SnSb alloy was revealed using operando X-ray diffraction and density functional theory calculations. Rather than separate reactions of Sn and Sb, the reaction of SnSb alloy proceeds through a synergetic sodiation/desodiation process via the mechanism: SnSb (crystalline) ↔ Na(Sn,Sb) (amorphous) ↔ Na9(Sn,Sb)4 (amorphous/low-crystalline) ↔ Na15(Sn,Sb)4 (crystalline).