In order to improve the irradiation damage resistance and irradiation hardening resistance of the welds of China Low Activation Martensitic steel (CLAM), the CLAM steel welds before and after heat treatment exposed to He+ with energy of 70-KeV to a dose of 1×1017 ions/cm2 at room temperature were analyzed by atomic force microscopy (AFM), scanning electron microscopy (SEM), grazing incident X-ray diffraction (GIXRD), tensile and nano-indentation techniques. The results show that micro defects such as voids are produced in different weld metals after ion irradiation, and the mechanical properties are reduced in varying degrees. After heat treatment, the weld of CLAM steel has the characteristics of smaller grain structure and higher grain boundary density, which hinders the mutual aggregation of micro defects. The defects formed in the weld are more evenly distributed and smaller in size. The weld always has better mechanical properties before and after irradiation. It is a feasible idea and method to refine weld grain through heat treatment process and improve the ability of weld to resist irradiation damage and irradiation hardening.