In this report, W⁶⁺ doping as a defect engineering strategy has been proposed to improve the electrochromic properties of NiO film. Further research was conducted to explore the electrochromic properties and the modified mechanism of W-doped NiO film. Compared to the pure NiO, W-doped NiO film exhibits improved electrochromic properties with significant optical modulation (61.56% at 550 nm), fast switching speed (4.42 s/1.40 s for coloring/bleaching), high coloration efficiency (45.41 cm²·C⁻¹) and outstanding cycling stability (no significant attenuation after 2000 cycles) in Li-based electrolytes. Density functional theory (DFT) calculations combined with the experimental results indicate that the improved electrochromic properties were due to enhanced the electronic conductivity and ion conductivity after the introduction of W⁶⁺. The charge capacity of W-doped NiO has also been improved, and it can function with WO₃ to achieve a high performance black electrochromic smart window (ECSW) by balancing charge. This work could advance the fundamental understanding of defect engineering as an effective strategy to boost the electrochromic properties of NiO anodic material, manifesting a significant development as a candidate counter electrode in high-performance black smart windows.