Deep heavy oil reservoirs, as critical replacement resources, are challenged by high crude oil viscosity, poor fluidity, and low efficiency of conventional water flooding. To address the unclear understanding of the oil enhancement mechanism by high-rate and large-volume injection of viscosity-reducing systems in deep heavy oil reservoirs, this study systematically investigated the influence mechanisms of high-rate and large-volume viscosity-reducer injection on pore structure, emulsification characteristics, and oil displacement efficiency in cores, using CT scanning and microscopic etched-model displacement experiments. The results indicated that high-rate and large-volume injection mainly improved reservoir physical properties by altering the distribution of pore radii and throat widths in the cores, and the effect of injection rate on pore structure was significantly greater than that of injection volume. High-rate injection enhanced the shear effect in porous media, which facilitated the emulsification of crude oil by the viscosity reducer, leading to the formation of crude oil emulsions with smaller and more stable droplet sizes. When the injection rate was increased from 1 mL/min to 10 mL/min, the peak particle size of the produced fluid decreased from 40 μm to 8 μm. Under high injection rate conditions, the comprehensive oil recovery was increased by 30.04%, which was much higher than the 14.57% achieved under low injection rate conditions. The high-rate shear environment not only strengthened the emulsification effect but also enhanced the capability of fluids to strip and carry wall-adhered oil films. The stable fine emulsion droplets regulated the water?oil mobility ratio through the Jamin effect, promoted the diversion of displacement fluid flow, and expanded the sweep volume. Based on the synergistic effect of enhanced flow capacity and improved emulsification, a three-dimensional oil enhancement mechanism of "flow?emulsification?sweep" was established, which effectively improved the crude oil recovery in deep heavy oil reservoirs. The study of this mechanism can provide theoretical guidance for the cost-effective development of deep heavy oil reservoirs.