Abstract: CO2 laser welding was used to weld zirconium plates of 5 mm thickness under the condition of additional protective gas being setbehind the side-blown gas, and the microstructure and mechanical properties of welded joint were studied by optical microscopy(OM), X-ray flaw detector, tensile testing and microhardness measuring. The results show that the weld seam was faint yellow without obvious porosity and crack, and met the gas shielding requirements for industry. The microstructure of the base metal, weld seam and heat affected zone were equiaxed α phase, needle α′ martensites and lath and needle martensites, respectively. The microhardness of the weld seam zone and base metal were 200 HV and 155 HV , respectively, and the microhardness decreased with the distance from the weld seam to the base metal. The tensile strength of weld seam was higher than that of base metal.