Photocatalytic degradation has been considered as an eco-friendly method for the removal of harmful antibiotics. Considering that the fundamental aspect of antibiotic photodegradation is a free radical reaction on the catalyst surface, boosting the generation of surface active free radicals is a direct and effective approach to promote the surface antibiotic oxidation reaction. However, rational regulation of the surface free radicals of photocatalysts is still a big challenge, and has also been rarely investigated. Herein, surface defect engineering was employed to introduce two different surface defect structures (i.e., two-coordinated nitrogen vacancies (N_2C) on g-C₃N₄ and oxygen vacancies on TiO₂) on the surface of a g-C₃N₄/TiO₂ Z-scheme photocatalyst by one-step hydrogenation treatment, aiming to effectively modulate the surface active free radicals that directly dominate the surface redox reaction. The resulting defect-rich hydrogenated g-C₃N₄/TiO₂ (H-CN/TiO₂) Z-scheme heterojunction produced much more ˙OH and ˙O₂⁻ free radicals on the surfaces of TiO₂ and g-C₃N₄, respectively, which would greatly facilitate the surface redox reactions in photocatalytic processes. As expected, the H-CN/TiO₂ Z-scheme heterojunction exhibited obviously improved activity in photodegradation of tetracycline hydrochloride compared with those of defect-free photocatalysts. Meanwhile, the effect of defect structures on the surface free radicals of the H-CN/TiO₂ photocatalyst was well explored and the photocatalytic mechanism was also discussed. This work paves the way for the regulation of free radical formation via surface defect engineering to further improve the photocatalytic activities of Z-scheme heterostructures.