It is still a great challenge to improve the diffusion dynamics of Na+ throughout the Na3V2(PO4)3, one of the most promising cathodes for sodium ion batteries. In this work, we present a facile doping strategy to tackle this problem by co-doping the Na+ with K+ and Ca2+. Na3-3xKxCaxV2(PO4)3/C (x = 0, 0.05, 0.07 and 0.09) composites are prepared by a sol-gel method combined with freeze-drying and high-temperature calcination processes, and the composites are characterized by XRD, XPS, SEM, TEM and electrochemical measurements. The results indicate that co-substitution of K+ and Ca2+ for Na in Na3V2(PO4)3 enlarges the cell volume of Na3V2(PO4)3 and is beneficial for improving electrochemical performance. All co-doped Na3V2(PO4)3/C composites have better electrochemical performance than the pristine Na3V2(PO4)3/C composite. The optimal doping compositions are found to be Na2.79K0.07Ca0.07V2(PO4)3/C, displaying reversible capacities of 110.2, 92.7 and 83.6 mAh·g-1 at the current densities of 0.1, 1, and 10C (1180 mA·g-1), respectively, in the voltage range of 2.5–4.0 V. The Na2.79K0.07Ca0.07V2(PO4)3/C exhibits capacity retentions of 91% at 1C after 50 cycles and 83% at 10C after 150 cycles, while Na3V2(PO4)3/C composite displays capacity retentions of 88.5% at 1C after 50 cycles and 26.5% at 10C after 150 cycles, respectively. The improved electrochemical performance of co-doped Na3V2(PO4)3/C is ascribed to enhanced Na+ diffusion after the co-substitution of K+ and Ca2+ for Na in Na3V2(PO4)3/C.