Abstract: Laser penetration welding of type-201 stainless steel and type-5052 aluminium alloy was investigated with nickel foil as the interlayer.Lap joint was applied with steel sheet on the top and aluminium sheet at the bottom.The effect of heat input on weld forming was studied, and the mechanical properties of the joints and weldability of stainless steel and aluminium alloy with addition of nickel foil interlayer were analyzed.The ratio of weld concaved width to penetration was put forward to represent the sensitivity of stress concentration.It was found that increasing heat input resulted in aggravating weld spatter and increasing of weld width, weld concaved width and penetration.The penetration depth of the weld into aluminium alloy had significant influence on the mechanical properties of the joint.The addition of nickel foil interlayer improved the mechanical properties of the resultant joints and extended the range of operable welding parameters.