The structure changes of iron disulfide (FeS2) during discharge/charge process were investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and synchrotron-based X-ray adsorption near the edge structure (XANES). Although the cyclability of the FeS2/Na battery is poor, FeS2/Na battery has a large first discharge capacity of 758 mA h g−1. It corresponds to 85% of the theoretical capacity based on the 4.0 Na reaction per FeS2 (893 mA h g−1). According to the Fe K-edge XANES spectra of discharged FeS2 pellets, there were no clear traces of iron valence change. However, the clear S K-edge jump shifted to lower energy with increasing Na content in FeS2. This is because the binding energy of S 1s electrons decrease as the oxidation number of S decreases, suggesting that the charge neutrality of the cathode after 2.0 Na discharge is kept by an S valence change from (S–S)2− to S2−. Moreover, Na2S diffraction peaks were detected at the 3.0 Na discharged state as a trace of Na conversion reaction in the FeS2 cathode.