An external magnetic field was firstly been applied to laser welding of Al/Ti alloys. Microstructure and tensile strength for Al/Ti laser-welded joint under external magnetic field were investigated. The results indicate that the strength of Al/Ti joint is improved by 44.4% via application of an external vertical magnetic field with an intensity of 120 mT. The enhancement of strength is attributed to the diminishment of intermetallic compounds, that is, the volume of TiAl3 is decreased from 10.69 to 6.44%. It is deemed that, on the one hand, the induced Lorentz force via the interaction between horizontal fluid flow and perpendicular magnetic field suppresses the horizontal convection, leading to the decreased thickness of intermetallic compounds. On the other hand, the thermoelectric magnetic convection contributes to the heat and solute transfer at the Al/Ti interface, leading to the redistribution of elements with disordered and decreased intermetallic compounds.