Abstract: CMT welding was carried out to weld-braze lap joint of 5052 aluminum alloy to galvanized Q235 steel using ER4043 filler wire. The effect of heat input on the microstructure and mechanical properties of the joint was investigated. The results show that the weld width, grain size of heat-affected zone(HAZ), hardness and thickness of interfacial layer increased with the increasing of heat input. The lap joints broke initially in the HAZ of aluminum alloy in tensile test due to grain coarsening. The weld zone mainly consisted of dendritic α-Al matrix and intergranular Al-Si eutectic, both of which grew to the weld center perpendicular to interfacial reaction layer and fusion line.The thickness of interfacial reaction layer in the brazed zone was around 2.55-6.86 μm. The interfacial reaction layer on aluminum side was uneven and consisted of FeAl₃ intermetallic compound(IMC), while that on steel side was uneven and mainly composed of Fe₂Al₅(with lower heat input) or FeAl₂ and FeAl(with higher heat input).