Abstract: The dissimilar metals of AZ91D alloy/Q345 steel were welded by MIG welding process,and the effects of welding current and welding speed on the microstructures and tensile strength of welded joint were studied. The results show that,at different welding currents,the weld zone is made of equiaxed grain,and with the increase of welding current,the grain size of weld zone increases gradually,the tensile strength of welded joint increases firstly and then decreases,and the maximum value of 144 MPa was obtained at the welding current of 130 A. With the increase of welding speed,the tensile strength of welded joint increases firstly and then decreases,and the minimum value of 38 MPa was obtained at the welding speed of 750 mm/min. MIG welding process can be used to weld AZ91D alloy/Q345 steel dissimilar metals at welding current of 130 A and welding speed of 600 mm/min.