Abstract: The mechanisms of grain refinement in Ti5321G alloy induced by an ultrasonic energy field (UEF) during laser powder direct energy deposition (LP-DED) were systematically investigated, focusing on the interplay between recrystallization in high-temperature solid deposition layers and ultrasonic cavitation-acoustic streaming effects during molten pool solidification. A novel experimental design was developed to decouple these mechanisms by creating four distinct UEF-action zones (N, S, L, S+L) within a single-pass multilayer sample. This approach enabled direct comparison of UEF’s dual effects: recrystallization in solidified layers and ultrasonic cavitation-acoustic streaming effects in liquid pools. The results demonstrated that UEF significantly refined microstructures, reducing average grain size by up to 64.1% (from 399.6 ± 28.6 μm to 143.1 ± 16.1 μm) in the S+L zone, while promoting columnar-to-equiaxed transition, with equiaxed grain probability increasing from 11.1% (without UEF) to 53.8%. Texture intensity was reduced by ~52.4%, and mechanical properties were enhanced, achieving a 6.2% increase in yield strength (702.0 ± 10.6 MPa) and 31.7% improvement in elongation. Crucially, this work reveals the synergistic effect of UEF’s dual-action mechanisms, where recrystallization and cavitation-acoustic streaming collectively enable nonlinear grain refinement. The study provides a groundbreaking strategy for microstructure control in additive manufacturing, eliminating the need complex post-processing, thereby advancing the industrial application of high-performance titanium components.