The development of low-cost and long-lifespan sodium-ion battery (SIB) cathode materials is crucial for large-scale energy storage. Iron-based phosphate cathode materials have attracted significant attention in recent years for their high theoretical capacity, excellent structural stability and rich resources. Here, a series of Na₄Fe_xP₄O_12+x/C (x=2.6-3.3) electrode materials are prepared using Sol-Gel technique and thermal treatment process. Effect of the phase structure on electrochemical performance of Na₄Fe_xP₄O_12+x/C electrode materials is investigated. It is found that three phases, including Na₂FeP₂O₇ (NFPO), Na₄Fe₃(PO₄)₂P₂O₇ (NFPP) and NaFePO₄ (NFP), mainly exist in the Na₄Fe_xP₄O_12+x/C system. Among Na₄Fe_xP₄O_12+x/C electrode materials, Na₄Fe_3.1P₄O_15.1/C electrode material with the highest content of NFPP phase possesses rapid electronic and sodium-ion conduction characteristics, thereby exhibiting the optimal electrochemical performance. As a result, the SIB equipped with Na₄Fe_3.1P₄O_15.1/C electrode material shows high reversible capacity, with a discharge specific capacity of 102.8 mAh·g^-1 at a current density of 0.1C (1C=129 mAh·g^-1), as well as capacity retention of 88.7% after 700 cycles. Furthermore, the as-assembled battery exhibits excellent rate performance with a discharge specific capacity of 61.5 mAh·g^-1 at a current density of 5C.