Abstract: The titanium alloys produced by laser directed energy deposition often suffer from coarse columnar grains, internal pores and significant residual tensile stress, which collectively limit the mechanical properties and service reliability. In this work, synchronous ultrasonic vibration assisted laser directed energy deposition was developed to prepare commercial pure titanium thin-walled structures, the grain refinement and defect suppression mechanisms were studied in detail. It was found that different ultrasonic amplitudes affect the range of bubble movement. When the ultrasonic amplitude is 20 μm, the bubbles can achieve efficient escape. The applied ultrasonic vibration fundamentally alters melt pool dynamics through acoustic streaming effect. Compared to conventional laser directed energy deposition, this effect actively transported bubbles to the surface, resulting in porosity reduced by 47.7% in the middle area of the samples. Meanwhile, the ultrasonic vibration modified the thermal field, obviously reducing the temperature gradient at the solidification front while increasing the solidification rate. It significantly promoted the random distribution of crystal orientation, achieved the columnar-to-equiaxed grains transition, and the overall microstructure of material was improved. These changes significantly mitigate residual tensile stress from 288.7 MPa to 49.3 MPa. Defect suppression, microstructure refinement and residual stress reduction increase yield strength from 408.5 MPa to 510.9 MPa (increased by 25%), the maximum uniform strain from 5.1% to 9.9%. The results demonstrated that synchronous ultrasonic vibration assistance was an effectively strategy for the integrated regulation of solidification microstructure and defect quantities, offering a potential pathway to enhance the quality and reliability of additively manufactured titanium components.