The present work reports a novel CdS/KTa0.75Nb0.25O3 (KTN) composite photocatalyst which was synthesized via a facile deposition method. The photocatalytic reaction in Na2S solution indicated that the as-synthesized composite presented excellent performance in water splitting under simulated sunlight and visible light. A thorough investigation was performed to reveal the origin of the high performance. X-ray diffraction (XRD), Raman, X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), and Transmission electron microscopy (TEM) experiments proved that CdS nanopartilces were uniformly dispersed on the surface of KTN naocubes. UV-visible diffuse reflection spectroscopy (DRS) analysis indicated that the addition of CdS improved the ability to absorb visible light. N2-adsorption analysis showed that the difference in surface area of the CdS/KTN composites was very small. Photoluminescence (PL) spectroscopy, electrochemical impedance spectroscopy (EIS), and transient photocurrent response (PC) analyses suggested that the added CdS greatly elevated the charge separation efficiency, which was considered as the key character of the composite. On the basis of the characterization results and the band structure of the two semiconductors, it is deduced that the CdS/KTN composite works according to a type-II mechanism under simulated sunlight. The optimal sample demonstrated a H2-generation rate of 1252 μmol·g-1·h-1, which is 260 and 48 times higher than that of KTN and CdS, respectively. Under visible light, photosensitization mechanism works in the composite. The synergetic effect of CdS and KTN in H2 production was also observed. Meanwhile, the composite also presented high photocatalytic stability. Considering the high activity and stability, the CdS/KTN may have potential application in photocatalytic H2 generation.