Abstract: Addressing the engineering demands for integrating additively manufactured titanium alloys with traditional plates, this study investigates the effects of laser power on the microstructural evolution and mechanical properties of dissimilar titanium alloy joints between TA17 plates and additively manufactured TC4 under a 1 kPa negative pressure environment. Three laser power levels—1500 W, 2500 W, and 3500 W—were selected for butt welding. The macroscopic morphology, phase composition, microhardness, and room-temperature tensile properties of the joints were systematically characterized. The results indicate that all joints consist primarily of the α-Ti phase, with no harmful oxides or new phases detected. As the laser power increases, the weld penetration depth and the width of the heat-affected zone (HAZ) increase significantly, while the microhardness gradient transitions more gradually. Mechanical testing reveals that the joint strength is constrained by the weaker TA17 side, exhibiting a "weakest link" effect. Among the three welding samples, the joints welded at 1500 W demonstrate the optimal comprehensive performance, achieving an ultimate tensile strength of 760 MPa and an elongation of 9%, thereby realizing the best balance between strength and ductility. This research provides a solid process foundation for the high-quality joining of titanium alloy structural components fabricated via different manufacturing processes.