The mixing characteristics of two initially stratified high-viscosity fluids with a free surface under forced vertical vibration were studied experimentally and numerically. The flow characteristics and dynamics of the free surface and interface between liquids were analyzed and the effects of the vibration parameters on the mixing process were evaluated. The degree of mixing was determined by the frequency and amplitude of vibration. Effective mixing could not be achieved by merely using the deformation of the free surface because of the low fluidity of high-viscosity fluids. However, an increase in vibration intensity disintegrated the free surface and interface, thereby significantly promoting the mixing of fluids and improving the mixing efficiency.