Fe_xMo_1–xS₂ with an expanded interlayer spacing of 0.75 nm was prepared via a simple solvothermal method. The larger interlayer spacing enhances the rate of Na⁺ diffusion during initial cycle. Fe_xMo_1–xS₂ as an anode for sodium ion batteries exhibits a high capacity of 285 m A·h/g at 0.1 A/g after 100 cycles and an excellent rate capability of 178 m A·h/g at 5 A/g. The fresh and cycled electrodes were characterized by in-situ X-ray photoelectric spectroscopy and transmission electronic microscopy to investigate electrochemical reaction mechanism of Fe_xMo_1–xS₂ during cycling. The results indicate that the irreversible conversion reaction of Fe_xMo_1–xS₂ with Na⁺ results in the formation of main products of Fe–Mo alloy and S.