Creating high-density durable bifunctional active sites in an air electrode is essential but still challenging for a long-life rechargeable zinc-air battery with appealing power density. Herein, we discover a general strategy mediated by metastable rock salt oxides for achieving high-density well-defined transition-metal nanocrystals encapsulated in N-doped carbon shells (M@NC) which are anchored on a substrate by a porous carbon network as highly active and durable bifunctional catalytic sites. Small-size (15 ± 5 nm) well-dispersed Co2Fe1@NC in a high density (metal loading up to 54.0 wt %) offers the zinc-air battery a record power density of 423.7 mW cm-2. The dual protection from the complete graphitic carbon shells and the anchoring of the outer carbon network make Co2Fe1@NC chemically and mechanically durable, giving the battery a long cycling life. Systematic in-situ temperature-dependent characterizations as well as DFT modeling rationalize the rock salt oxide-mediated process and its indispensable role in achieving high-density nanosized M@NC. These findings open up opportunities for designing efficient electrocatalysts for high-performance Zn-air batteries and diverse energy devices.