The main obstacles of metallurgical bonding and reliable joining of Mg/Ti dissimilar joint are their nonreactive and immiscible characteristics. To overcome those problems and expand their application, laser lap joining of Mg/Ti with Al-rich Mg based wire is carried out. Mg/Ti interface is observed with scanning electron microscope, and the role of Al element in interfacial diffusion and bonding mechanism is investigated based on the result of calculation of Miedema binary and Toop ternary thermodynamic model. The results demonstrate that an ultra-thin reaction layer is observed at the Mg/Ti interface with the help of Al elements from filler. The element line scan results show that enrichment of Al element is evidenced at the interface, indicating metallurgical bonding of Mg and Ti. Thermodynamic calculation result reveals Al- Ti intermetallic compound has more driving force than other compounds, which indicates that it is the compound firstly precipitated from the liquid. In addition, Al chemical potential near the Ti side is lower than that at other places. Al segregation at the interface further causes the decrease of Al chemical potential, indicating the behavior of Al element from Mg fusion zone to the interface is uphill diffusion.