The research on laser welding and laser welding with filler wires of 3-mm thick C18000 copper alloy plates is conducted. The microstructures, compositions and mechanical property of welds are analyzed emphatically. The results show that, in the case of direct laser welding, the growth of weld grains appears, the weld microhardness is 90 HV, and the weld strength is only 313 MPa. While in the case of laser welding with ERNiCu-7 filler wires, the mass fraction of Ni from top to bottom of the welds drops from 31% to 10%, the weld structure is single-phase solid solution, the weld microhardness increases to 130-180 HV, and the weld strength increases to 391 MPa. These performances are superior to those for the direct laser welding.