The ability to regulate charge separation is pivotal for obtaining high efficiency of any photoelectrode used for solar fuel production. Vacancy engineering for metal oxide semiconductor photoelectrode is a major strategy but has faced a formidable challenge in bulk charge transport because of the elusive charge self-trapping site. In this work, a new deep eutectic solvent to engineer bismuth vacancies (Bi ) of BiVO photoanode is reported; the novel Bi can remarkably increase the charge diffusion coefficient by 5.8 times (from 1.82 × 10 to 1.06 × 10 cm s ), which boosts the charge transport efficiency. Through further loading CoBi cocatalyst to enhance charge transfer efficiency, the photocurrent density of BiVO photoanode with optimal Bi concentration reaches 4.5 mA cm at 1.23 V vs reversible hydrogen electrode under AM 1.5 G illumination, which is higher than that of previously reported O engineered BiVO photoanode where the BiVO photoanode is synthesized by a similar procedure. This work perfects a cation defect engineering that enables the potential capability to equate the charge transport properties in different types of semiconductor materials for solar fuel conversion.