The butt-joint laser welding of magnesium and titanium alloys is achieved by shifting laser pulses on the magnesium side and adding copper interlayers. The diffusion of elements and reaction characteristics at the interface are investigated. Furthermore, the effect of the thickness of the copper interlayer on the microstructure and mechanical properties of the joint is investigated. In addition, the primary causes of joint fractures are analyzed. The results show that the copper interlayer improves the microstructure at the weld/titanium interface and increases the content of Ti-Cu compounds near the interface. As the thickness of the copper interlayer increases, the thickness of the Ti₂Cu reaction layer at the interface also gradually increases and becomes continuous. Moreover, the fracture position of the joint moves from the interface reaction layer to the weld zone. When the thickness of the copper interlayer reaches 30 μm, the tensile strength of the joint reaches 121 MPa.