Viscosity-reducer foam flooding combines the advantages of emulsification of viscosity-reducers and selective blocking of foam to further enhance oil recovery in deep low-permeability heavy oil reservoirs during the later development stages. The suitable concentration of viscosity-reducing foam agents were selected according to the viscosity-reducing effect，foaming performance and foam stability of the agents by laboratory experiments. Meanwhile，the characteristics and recovery effects of viscosity-reducer foam flooding under different displacement modes were compared by single-core displacement experiments， and the diverting capacity of viscosity-reducing foam at various permeability ratios were investigated by parallel core experiments，and thus the enhanced oil recovery （EOR）mechanisms of viscosity-reducer foam flooding ware defined. The results demonstrate that the viscosityreducing agents facilitate the emulsification and viscosity reduction of heavy oil， while the foams block large pore throat and inhibit N₂ channeling during viscosity-reducer foam flooding. Combination of the both can improve the sweep coefficient and displacement efficiency，increase displacement pressure differential，and lower the water cut. Furthermore，the viscosity-reducer foam flooding can enhance the oil recovery by an additional 13% based on the viscosity-reducer flooding. In heterogeneous conditions，viscosityreducer foam flooding decreases the high-permeability core channeling to direct the flow towards low-permeability cores，thereby exerting an emulsification and viscosity-reducing effect. Finally，this expands the swept volume and simultaneously enhances the displacement efficiency. While the viscosity-reduce foam flooding significantly enhance the oil recovery of deep low-permeability heavy oil reservoirs by optimizing the oil flow distribution，enhancing emulsification，and reducing heavy oil viscosity， thus providing an effective strategy for efficient development of deep heavy oil reservoirs.