This paper is designed for elevating the photocatalytic H2-evoultion performance of g-C3N4 through the modification of AgNbO3 nanocubes. Via the microwave heating method, g-C3N4 was in-situ formed on AgNbO3 surface to fabricate a close contact between the two semiconductors in forty minutes. X-ray diffraction (XRD), Fourier transform-infrared (FT-IR), X-ray photoelectron spectroscopy (XPS) experiments were performed to confirm the binary structure of the synthesized AgNbO3/g-C3N4 composite. N2-adsorption and visible diffuse reflection spectroscopy (DRS) analyses indicated that the addition of AgNbO3 to g-C3N4 showed nearly negligible influence on the specific surface area and the optical property. Photoluminescence (PL) spectroscopy experiment suggested that the AgNbO3/g-C3N4 displayed reduced PL emission and longer lifetime of photoexcited charge carriers than g-C3N4, which could be ascribed to the suitable band potential and the intimate contact of g-C3N4 and AgNbO3. This result was also confirmed by the transient photocurrent response experiment. The influence of the enhanced charge separation was displayed in their photocatalytic reaction. AgNbO3/g-C3N4 sample showed enhanced performance in photocatalytic H2-generation under visible light illumination. The H2-evolution rate is determined to be 88 μmol·g-1·h-1, which reaches 2.0 times of g-C3N4. This study provides a feasible and rapid approach to fabricate g-C3N4 based composite.