Three-dimensional ultrasonic waves with amplitudes of 14, 18, and 22 μm, respectively, are used during the solidification of (FeCoNiCrMn)₉₂Mo₈ high-entropy alloy, and its microstructural evolution and mechanical property are investigated in this work. Under static condition, the solidification microstructure is composed of primary γ phase dendrites with FCC structure and stripe-shaped σ phase with tetragonal structure. As the ultrasonic amplitude increases, the mean transient cavitation intensity rises to trigger off 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 the average acoustic streaming velocity increase simultaneously, which accelerates atomic diffusion at the liquid/solid interface, reducing Cr content in the primary γ phase from 18.6% to 13.1% and Mo content from 6.8% to 3.4% (atomic percent). This atomic redistribution subsequently causes the liquid composition to approach the eutectic point and facilitate the formation of (γ + σ) eutectic, which accounts for more than 50% volume fraction. The two eutectic phases exhibit a semi-coherent interface relationship characterized by [110]_γ//[110]_σ and $(1\bar1\bar 1) $_γ//$(\overline110) $_σ. Furthermore, due to the gradual enrichment of Cr atoms in the remaining liquid phase, a small quantity of metastable μ phases with Cr content up to 62.3% form in the final microstructure. The maximum compressive yield strength of the ultrasonically solidified microstructure reaches 876.2 MPa, almost twice that of static solidification microstructure, and the compressive strain reaches 33.2%. The formation of (γ + σ) eutectic is the main factor that increases the yield strength of alloy by 527.1 MPa.