Metallic‐phase selenide molybdenum (1T‐MoSe2) has become a rising star for sodium storage in comparison with its semiconductor phase (2H‐MoSe2) owing to the intrinsic metallic electronic conductivity and unimpeded Na+ diffusion structure. However, the thermodynamically unstable nature of 1T phase renders it an unprecedented challenge to realize its phase control and stabilization. Herein, a plasma‐assisted P‐doping‐triggered phase‐transition engineering is proposed to synthesize stabilized P‐doped 1T phase MoSe2 nanoflower composites (P‐1T‐MoSe2 NFs). Mechanism analysis reveals significantly decreased phase‐transition energy barriers of the plasma‐induced Se‐vacancy‐rich MoSe2 from 2H to 1T owing to its low crystallinity and reduced structure stability. The vacancy‐rich structure promotes highly concentrated P doping, which manipulates the electronic structure of the MoSe2 and urges its phase transition, acquiring a high transition efficiency of 91% accompanied with ultrahigh phase stability. As a result, the P‐1T‐MoSe2 NFs deliver an exceptional high reversible capacity of 510.8 mAh g−1 at 50 mA g−1 with no capacity fading over 1000 cycles at 5000 mA g−1 for sodium storage. The underlying mechanism of this phase‐transition engineering verified by profound analysis provides informative guide for designing advanced materials for next‐generation energy‐storage systems. By adopting a novel plasma‐assisted doping‐triggered phase‐transition engineering, stabilized P‐doped metallic phase selenide molybdenum (MoSe2) nanoflower composites (P‐1T‐MoSe2 NFs) with expanded interlayer spacing, metallic electronic conductivity, facilitated Na+ adsorption, and reduced Na+ diffusion barrier are fabricated for high‐performance sodium storage. The underlying mechanism analysis provides informative guide for designing advanced materials for next‐generation energy‐storage systems.