Heterostructured photocatalysts based on g-C3N4 and TiO2 represent a promising kind of photocatalyst in environmental fields, but the synthesis methods are always complex and not green. In the present paper, a facile and green one-step calcination procedure at lower temperature (450 °C) with the assistance of water is developed to synthesize a visible-light-active TiO2@g-C3N4 heterostructured photocatalyst, which shows higher visible-light-driven activity (k = 0.014 min-1) than pure g-C3N4 (k = 0.0036 min-1) and TiO2 (k = 0.0067 min-1) for methyl orange degradation. Excellent performance (over 90% conversion) was also observed for the removal of rhodamine B, phenol, and Cr(VI) under visible light. The heterostructured photocatalyst showed favorable reusability, preserving 86% of its activity after five successive cycles. A mechanism study demonstrates that the enhanced photocatalytic activity results from the efficient separation of the photo-generated charge carriers through the intimate interface between the two semiconductors based on their appropriate band structures and light-induced mechanism. The heterostructured photocatalyst will certainly find wide applications in the treatment of various toxic pollutants in wastewater using abundant solar energy. Furthermore, this facile and green procedure and the proposed synergistic mechanism will provide guidelines in designing other g-C3N4 based organic-inorganic composite photocatalysts for various applications.