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.