Laser lap welding of 0.2 mm-thickness 304 stainless steel and 0.2 mm-thickness T2 red copper ultra-thin sheets is conducted by using a fiber laser with a wavelength of 1070 nm. With the steel-copper-steel tri-layer lap welding method, good welds and wide process windows can be obtained. Black layered patterns parallel to the weld edge are observed in the weld microstructure. Energy spectral analysis results show that the content of copper at the weld bottom is higher than that at the weld top. The microhardness test results show that the lowest hardness zone is in the heat affected zone of the copper sheet, and the unilateral width of the heat affected zone is about 1 mm. The tensile test results show that the fracture occurs in the heat affected zone of the copper sheet, and the maximum tensile strength is 213 MPa.