Three-dimensional ultrasonic waves with amplitudes of 14, 18, and 22 μm were applied during the solidification of (FeCoNiCrMn)₉₂Mo₈ high-entropy alloy, and its microstructural evolution and mechanical property were investigated. Under static condition, the solidification microstructure was composed of primary γ phase dendrites with FCC structure and stripe-shaped σ phase with tetragonal structure. As the ultrasonic amplitude increased, the mean transient cavitation intensity rose to trigger a significant nucleation rate increase of the primary γ phase to 5.6×10¹² m^-3·s^-1, leading to the remarkable grain size reduction by two orders of magnitude. The maximum and average acoustic streaming velocity increased concurrently, which accelerated atomic diffusion at the liquid/solid interface, reducing Cr content in the primary γ phase from 18.6 at.% to 13.1 at.% and Mo content from 6.8 at.% to 3.4 at.%. This atomic redistribution subsequently caused the liquid composition approaching the eutectic point and facilitated the formation of (γ+σ) eutecticss, which took up more than 50% volume fraction. The two eutectic phases exhibited a semi-coherent interface relationship characterized by (110)_γ//(110)_σ and (1-1-1)_γ//(-110)_σ. Furthermore, due to the progressive enrichment of Cr atoms in the remaint liquid phase, a small amount of metastable μ phase with Cr content up to 62.3 at.% formed in the final microstructure. The maximum compressive yield strength of the ultrasonically solidified microstructure reached 876.2 MPa, which was nearly twice of that for static solidification microstructure, and the compressive strain reached 33.2%. The formation of (γ+σ) eutectics represented as the dominant factor to contribute an enhancement of 527.1 MPa to the alloy's yield strength.