This work was designed to synthesize an efficient photocatalyst for photocatalytic H-2 evolution via the decoration of carbon on an optimal KTa1-xNbxO3 solid solution. The C/KTa1-xNbxO3 composite was fabricated via a two-step hydrothermal process, and investigated by various techniques, including N-2 adsorption, X-ray diffraction (XRD), Raman, scanning electron microscopy (SEM), transmission electron microscopy(TEM), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), transient photocurrent response (PC) an electrochemical impedance spectroscopy (EIS). The catalytic performance of the composite was investigated via photocatalytic H-2 evolution and rhodamine B (RhB) degradation under irradiation of simulated sunlight. Results indicate that the formation of KTa1-xNbxO3 solid solution improves the bulk charge separation, and consequently leads to the high photocatalytic activity. KTa0.75Nb0.25O3 solid solution presents the fastest rate for H-2 evolution. The loading of a thin carbon layer on the KTa0.75Nb0.25O3 nanocube can further fasten the process. The optimal composite sample displays a H-2 evolution rate of 1776 mu mol . g(-1) . h(-1), which is 2.44 and 9.40 times that of KTa0.75Nb0.25O3 and KTaO3, respectively. The load carbon thin layer enhances the surface area and hinders the surface charge recombination via trapping the electrons, which is believed to be the main reason for the excellent performance. Additionally, the synthesized C/KTa0.75Nb0.25O3 composite also presents good photoactivity in RhB degradation. This work affords a practical approach for the design and preparation of highly efficient photocatalysts via bulk doping and surface modification simultaneously.