A series of Al-xSi-yGe filler metals (x=4–12 and y=10–40, wt%) were prepared, and the effect of Si and Ge on microstructure and melting characteristics of filler metals was studied. The thermodynamic model of Al-Si-Ge ternary alloy was established to analyze the phase formation mechanism of filler metals based on Miedema model, Tanaka model, and Toop equation. This research provided a basis for the composition optimization of filler metals and the analysis of metallurgical reaction process between filler metals and base materials. Results show that Al-Si-Ge alloy is composed of Al-Ge eutectic phase, Al-Si eutectic phase, and primary Si. Ge addition promotes the precipitation of primary Si. Ge is the main melting point depressant element of filler metals. With the increase in Ge content from 10wt% to 40wt%, the solid phase line of filler metals remains unchanged, whereas the liquidus temperature decreases from 567.65 °C to 499.96 °C. With the increase in Ge content of filler metal, Ge content in eutectic Si phase is increased, the endothermic peak of Al-Si eutectic reaction according to thermogravimetry curve becomes smoother, and Al-Si eutectic temperature is decreased. Ge addition can reduce the free energy of Al-Si alloy system. The lowest point of free energy is located on Al-Ge side. The eutectic Ge phase with the composition similar to pure Ge composition is the most likely to appear in the microstructure of filler metals, whereas the eutectic Si phase with the composition similar to pure Si composition is the least likely to appear. The thermodynamic calculation results are consistent with the experiment results.