To meet the demand for efficient joining of dissimilar materials in aerospace, this study explores interface regulation mechanisms during ultrasonic welding of TC4 titanium alloy to carbon fiber-reinforced polyphenylene sulfide (CF/PPS) composites. A multiscale physicochemical interface engineering strategy is proposed, combining silane coupling agent functionalization with laser microtexturing. We systematically examined how welding parameters, energy director design, and interface modifications affect joint performance. Composite modification through titanium-surface laser engraving and energy-director coatings shifted the interfacial bonding mechanism from mechanical interlocking to a mechanochemical synergy. Under shallow-depth constraints, the linear microtextures achieved optimal shear strength (32.03 MPa), representing a 70.2% increase over deep-depth conditions (18.82 MPa). Without depth restrictions, circular arrays showed superior stability, with an average shear strength of 24.26 MPa, 23.7% higher than grid patterns. Fractography revealed cohesive, interfacial, and mixed failure modes, clarifying the mechanisms behind the strength improvement. This work provides both theoretical and practical insights for dissimilar-material joining in aerospace applications.