Photocatalysis has been rapidly developed as a sustainable technology to decompose contaminants by using photogenerated carriers excited through light irradiation. Electrons for molybdenum trioxide (MoO3) semiconductor with wide band gap can be easily transferred to its conduction band via dye sensitization effect under visible light. However, MoO3 still suffers from poor photocatalytic ability for organic dyes due to the low energy level of the conduction band and the insufficient utilization of the induced electrons. In this study, molybdenum dioxide (MoO2) nanoparticles were decorated on the surface of MoO3 nanobelts without requiring an additional Mo source by using a simple in-situ hydrothermal method. In the reaction process, the partial MoO3 itself was reduced to metallic MoO2 nanoparticles, and the resulting intimate interface between MoO2 and MoO3 could accelerate the transfer of dye sensitization-induced electrons. The as-prepared MoO2/MoO3 nanocomposites exhibited extremely enhanced visible light photocatalytic activity for decomposing rhodamine B (RhB) with the assistance of H2O2. The mechanism for high-efficiency degradation was analyzed and explored by conducting theoretical calculations and designing further experiments. The high-efficiency degradation might be due to the synergistic effect caused by the well-matched energy band structure between dyes and MoO3, and the metallic MoO2 nanoparticles, which can accelerate the production of hydroxyl radical (center dot OH) from H2O2. center dot OH is a dominant reactive species for the degradation of RhB under visible light irradiation.