Cavitation is a very important hydrodynamic phenomenon in many scientific and engineering fields, such as acoustics, medicine, and hydraulics. The relationship between the physical characteristics of liquid media and the erosion status of materials in a cavitation field is a crucial concern for many researchers. In this study, we adopted underwater low-voltage discharge technology to generate cavitation bubbles. We studied the effect of the viscosity of liquid media on cavitation bubble dynamics at the mesoscale level by using high-speed photography and a transient stress test system to illustrate the mechanism and the characteristics of material erosion under viscosity changes at the macroscopic level. It was found that high liquid viscosity delays shrinkage of the cavitation bubble and increases the minimum volume to which the cavitation bubble shrinks. The shrinkage characteristics of a cavitation bubble in a solution with high viscosity can reduce the speed of micro-jet formation during the collapse of the cavitation bubble near a wall. In addition, they can delay the impact of the micro-jet on the wall surface and reduce the impact strength when the cavitation bubble collapses. The effect of viscosity on cavitation bubble dynamics at the mesoscale level can explain the erosion law of solid-wall surfaces in a cavitation field from a mechanical point of view. These conclusions present significant potential for cavitation application and prevention in the fields of ultrasonic cleaning, medicine, and hydraulic machinery.