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Abstract Sodium all‐solid‐state batteries (NaSSBs) with an alloy‐type anode (e.g., Sn and Sb) offer superior capacity and energy density compared to hard carbon anode. However, the irreversible loss of Na⁺at the alloy anode during the initial cycle results in diminished capacity and stability, impairing full‐cell performance. This study presents an easy‐to‐implement cathode presodiation strategy by employing a Na‐rich material to address these challenges. Leveraging the high theoretical capacity and suitable voltage window, Na₂S is chosen as the Na donor, which is activated by creating a mixed electron‐ion conducting network, delivering a high capacity of 511.7 mAh g⁻¹. By adding a small amount (i.e., 3 wt.%) of Na₂S to the cathode composite, a NaCrO₂|| Sn full cell demonstrated capacity improvement from 90.8 to 118.2 mAh g⁻¹(based on cathode mass). The capacity‐balanced full cell can thus cycle to more than 300 times with >90% capacity retention. This work provides a practical solution to enhance the full‐cell performance and advance the transformation from half‐cell to full‐cell applications of NaSSBs.