A novel SnAgCu composite solder with multi-phase and multi-scale hybrid reinforcement was developed. Initially, surface modification of nano-sized ZrO₂ and micro-sized tetra-needle-like ZnO whisker (T-ZnO_w) was performed using pyrolysis method. Subsequently, the modified ZrO₂ and T-ZnO_w were incorporated into Sn1.0Ag0.5Cu composite solders using an ultrasonic-assisted casting method. The microstructure evolution, interface between solder matrix and reinforcements, and mechanical properties were systematically investigated. The results indicated that the composite solder exhibited a high proportion of eutectic structures with minimal coarse intermetallic compounds. Furthermore, at the interface between the reinforcements and Sn1.0Ag0.5Cu, no gaps, micropores, or new phases were observed, while atomic inter-diffusion was detected. When the Zn/Zr molar ratio was set to be 2꞉3, the composite solder achieved an ultimate tensile strength of 35.9 MPa and an elongation of 31.4%, representing improvements of 30.5% and 47.4%, respectively, compared to plain Sn1.0Ag0.5Cu solder.