This study investigates the relationship between heavy oil viscosity and emulsification performance during emulsification-based viscosity reduction. Heavy oils with different viscosities were prepared and their emulsification behavior was systematically evaluated using minimum emulsification speed (Rmin), droplet morphology, particle size distribution, water separation rate, and oil–water interfacial tension as key indicators. The results reveal the existence of a critical emulsification viscosity of approximately 3,700 mPa·s under the experimental conditions (60 °C, oil–water ratio 3:7, surfactant concentration 1 wt%). When the viscosity decreases below this threshold, the minimum emulsification speed decreases significantly, the droplet size distribution becomes more uniform, and the emulsion stability improves markedly. Meanwhile, the oil–water interfacial tension decreases to the low interfacial tension regime (<1 mN·m−1), facilitating droplet deformation and breakup during emulsification. Mechanistic analysis indicates that the observed critical viscosity originates from the synergistic effects of viscosity-controlled droplet deformation, surfactant adsorption kinetics at the oil–water interface, and the interfacial films formed by asphaltenes and resins. These findings provide theoretical guidance for optimizing emulsification viscosity-reduction systems and defining the applicable viscosity range for heavy oil emulsification technologies.