β-C 3 N 4 is one of the most difficult phases to synthesize among the C 3 N 4 structures (α, β, cubic, pseudo cubic, and graphitic), and it has attracted marked attention because of its unique super hardness, optical properties, low density, good chemical resistance, and thermal stability. In this paper, a uniform β-C 3 N 4 layer is successfully prepared and coated on a TiO 2 nanotube substrate via a calcination treatment using ethylenediamine as the carbon nitride precursor. The as-obtained TiO 2 /C 3 N 4 exhibites a promising sunlight-driven overall water splitting ability, with 1026.7 μmol H2 g cat. −1 h −1 and a 446.4 μmol O2 g cat. −1 h −1 generation rates simultaneously realized. Importantly, the turnover frequency for O 2 in the photocatalytic splitting process reaches up to 4472.8 h −1 for the overall water splitting process, which is ~77.6 times that of the single β-C 3 N 4 system (57.6 h −1 ) for O 2 generation. In-situ Raman investigation demonstrates that N vacancy defects in β-C 3 N 4 served as the active site for O 2 generation from H 2 O, which is a hard nut to crack for C 3 N 4 all the time. • β-C 3 N 4 has been successfully prepared and coated on TiO 2 nanotubes under ambient pressure. • The photocatalyst has outstanding overall water decomposition activity. • This work proved that the N vacancy defect in β-C 3 N 4 is the active site for O 2 generation by in-situ Raman examination.