A novel KTa0.75Nb0.25O3 (KTN)/g-C3N4 composite photocatalyst was fabricated through microwave heating for realizing the efficient photocatalytic H-2 evolution. The energy-efficient preparation method allowed g-C3N4 to be formed in-situ on KTN surface in thirty five minutes. The binary constitution of the KTN/g-C3N4 composite was verified by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) experiments. UV-visible diffuse reflection spectroscopy (DRS) experiments suggested that the photoabsorption performance was increased after the introduction of KTN. N-2-adsorption analysis indicated that the addition of KTN slightly increased the surface area of g-C3N4. Photoluminescence (PL) spectroscopy, electrochemical impedance spectroscopy (EIS) and transient photocurrent response (PC) analyses confirmed that the KTN/g-C3N4 composite displayed longer lifetime of photoexcited charge carriers than g-C3N4, owing to the suitable band potentials and the close contact of KTN and g-C3N4. This property was believed to the key characteristic of the composite, which led to its excellent photocatalytic performance. Under simulated sunlight irradiation, the optimal KTN/g-C3N4 catalyst presented a photocatalytic H-2-generation rate of 1673 mu mol.g(-1).h(-1), 2.5 and 2.4 times higher than that of KTN and pure g-C3N4, respectively. Under visible light irradiation, the value was determined to be 86.2 mu mol.g(-1.)h-(1), which achieved 9.3 times that of g-C3N4.