Effect of trace amount of Ga2O3 addition on the changes of the wettability and spreadability of CsF-RbF-AlF3 flux and Zn-Al filler metal were investigated on 6063 aluminum alloy and Q235 low-carbon steel. The experimental results indicated that trace amount of Ga2O3 addition in the flux could obviously promote the Zn-Al filler metal to wet and spread on the surface of 6063 aluminum alloy and Q235 low-carbon steel, in which the optimum range of Ga2O3 was 0.001~0.003wt.%. According to the XRD results of the flux residua and the thermodynamic calculation, it could be concluded that the trace amount of Ga2O3 addition could enhance the solubility and reactivity of CsF-RbF-AlF3 flux to the surface oxides on 6063 aluminum alloy and Q235 low-carbon steel. On the other hand, Ga2O3 could also be reduced to Ga. Because of its skin effect, Ga could enrich on the interface of filler metal and base metal and effectively decreased its interfacial tension, which could improve the wettability and spreadability of the filler metal.