Sodium-ion batteries (SIBs) have been considered to be an appealing alternative as alternatives for Lithium-ion batteries (LIBs) on account of abundant resources and low prices. Different cathode materials have been extensively studied, especially layered oxides (P2 or O3). However, it is crucial to design an appropriate cathode material for SIBs with excellent comprehensive performance. Herein, a biphasic cathode material with P2 and O3 structures is successfully synthesized by Sn substitution in Na0.67Ni0.33Mn0.67O2, which is clearly proved by the XRD and HRTEM results. And Fullprof program refinement is used to determine the optimal phase ratio of P2 and O3. Besides, the XRD tests at different voltages illustrate that the cathode material has good reversibility and structural stability during charge and discharge. Due to the synergistic effect of P2 and O3 phase, when the phase ratio is about 1:1, the biphasic material P2/O3–Na0.67Ni0.33Mn0.57Sn0.1O2 exhibits the most outstanding electrochemical performance. The P2/O3–Na0.67Ni0.33Mn0.57Sn0.1O2 cathode delivers a high reversible initial capacity of 155.2 mA h g−1 during the voltage range from 2.0V up to 4.3V with an excellent initial Coulombic efficiency of 98.95%, as well as the satisfactory rate performances and cycling performance. GITT testing also demonstrates the DNa+ of biphasic material is more stable and conducive to the diffusion of sodium ions. These results will pave a significant way for designing cathode material for SIBs with different structure by the substitution of transition metal elements.