Abstract: The ultrasonic energy field (UEF)-induced grain refinement mechanisms in laser powder direct energy deposition-manufactured Ti5321G alloys were systematically investigated in this study. This study focused on the interplay between recrystallization in the high-temperature solid deposition layers and the 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 (without UEF-N, with UEF-S, with UEF-L, and with UEF-S + L) within a single-pass multilayer sample. This approach enabled the dual effects of UEF (recrystallization in solidified layers and ultrasonic cavitation-acoustic streaming effects in liquid pools) to be directly compared. The UEF significantly refined the microstructures, reducing the average grain size by 64.2% (from (399.6 ± 28.6) to (143.1 ± 16.1) μm) in the with UEF-S + L zone, while promoting columnar-to-equiaxed transition, with the equiaxed grain probability increasing from 11.1% (without UEF) to 53.8%. The texture intensity was reduced by approximately 52.4% and the 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 study revealed the synergistic effect of the dual-action mechanisms of UEF, where recrystallization and cavitation-acoustic streaming collectively enabled non-linear grain refinement. This study provides a strategy for microstructural control in additive manufacturing, eliminating the need for complex post-processing and thereby advancing the industrial application of high-performance titanium components.