Abstract: The lap joining of magnesium alloy AZ31B to steel Q235 by laser-tungsten inert gas (TIG)welding was investigated. The effect of welding parameters on the tensile strength was studied, including laser power, welding speed, laser defocusing amount and current of TIG.It is indicated that the tensile strength of joint increases when the laser power gets higher.But the tensile strength of joint decreases when welding speed gets higher.The current of TIG has nothing to dowith the tensile strength.A weld with ideal appearance and good mechanical properties can be obtained with proper process parameters.In the welding process, laser power plays an important role in joining magnesium alloy with steel.