Gliding arc plasma is explored to synthesize C-doped TiO2 photocatalysts via tuning the ratio of O2 to titanium tetraisopropoxide (TTIP) for the first time. XRD and HRTEM characterizations confirm that the samples synthesized at O2/TTIP ≥ 18 are highly crystalline. When the O2/TTIP ratio ranges from 18 to 36, the as-synthesized samples with ~70% of the weight fraction of anatase, 40–50 m2 g−1 of specific surface area and ~2.7 eV of band energy gap, do show a greatly enhanced visible-light absorption and high activity for methylene blue (MB) degradation under visible-light irradiation (λ > 420 nm). The gray sample synthesized at O2/TTIP = 27 has the largest relative content of Ti3+ and oxygen vacancy (□) (Ti3+/□/Ti = 0.7/0.35/1) and exhibits the highest visible-light activity.