Phosphorus doping is an effective strategy to simultaneously improve the electronic conductivity and regulate the ionic diffusion kinetics of TiO₂ being considered as anode materials for sodium ion batteries. However, efficient phosphorus doping at high concentration in well-crystallized TiO₂ nanoparticles is still a big challenge. Herein, we propose a defect-assisted phosphorus doping strategy to selectively engineer the surface structure of TiO₂ nanoparticles. The reduced TiO_2–x shell layer that is rich in oxygen defects and Ti³⁺ species precisely triggered a high concentration of phosphorus doping (∼7.8 at. %), and consequently a TiO₂@TiO_2–x-P core@shell architecture was produced. Comprehensive characterizations and first-principle calculations proved that the surface-functionalized TiO_2–x-P thin layer endowed the TiO₂@TiO_2–x-P with substantially enhanced electronic conductivity and accelerated Na ion transportation, resulting in great rate capability (167 mA h g^–1 at 10 000 mA g^–1) and stable cycling (99% after 5000 cycles at 10 A g^–1). Combining in/ex situ X-ray diffraction with ex situ electron spin resonance clearly demonstrated the high reversibility and robust mechanical behavior of TiO₂@TiO_2–x-P upon long-term cycling. This work provides an interesting and effective strategy for precise heteroatoms doping to improve the electrochemical performance of nanoparticles.