The off-stoichiometric iron-based phosphate (Na_3.12Fe_2.44(P₂O₇)₂, denoted as Na_3.12) as a low cost and high structure stability cathode material has been widely studied for sodium-ion batteries (SIBs). However, the lower theoretical specific capacity (117 mAh·g⁻¹) has seriously limited its practical application. In this work, we incorporate varying proportion of sodium-iron phosphate (NaFePO₄) into the Na_3.12 to form a series of new high specific capacity mixed-phosphates Na_3.12+xFe_2.44+x(P₂O₇)₂(PO₄)_x cathode materials for SIBs. After optimizing the introduction amount of NaFePO₄ into Na_3.12, the practical reversible of Na_3.12+xFe_2.44+x(P₂O₇)₂(PO₄)_x increased from 92 to 125.2 mAh·g⁻¹. The nano-size Na_5.12Fe_4.44(P₂O₇)₂(PO₄)₂ cathode material shows a reversible specific capacity of 125.2 mAh·g⁻¹ at 0.1 C in SIBs. Even at 60 C, it still exhibits a reversible specific capacity of 93.3 mAh·g⁻¹ and keeps a capacity retention ratio of 87% after 3000 cycles at 20 C. Thereby, we present a novel approach to design a series of off-stoichiometric mixed-phosphates cathode materials for SIBs.