The laser welding-brazing experiment of Ti/Al dissimilar alloys was performed with rectangular spot CO2 laser as heat resource and AlSi12 as the filler wire. It is found that the porosity in the seam is a main factor for weak joint. The results indicate that the gasification of element Mg causes the formation of porosity during the welding. The offset of the laser beam toward Ti alloy and heat input markedly influence the formation of the porosity. In general no porosity is observed in the seam when the length of claviform intermetallic compound is less than 10 μm. Moreover, the microstructure in the seam plays a key role in the diameter of the pore.