Abstract: Welding is an important mean of assembling the cladding layer of fusion reactor. The study of ion irradiation effect of weld is of great significance for improving the service life of nuclear reactor core components. In order to compare the ion irradiation effect of China low activated martensitic steel (CLAM) welded weld and tempered weld, the welds were subjected to He⁺ irradiation at room temperature of 150 keV. X-ray diffraction (XRD) and continuous stiffness nano-indentation technique (CSM) were used to study the changes of the crystal structure and the mechanical properties of the weld in two states before and after irradiation. The results show that under 15dpa ion irradiation, the peak width widens and the diffraction angle shifts to small angle in the weld, moreover, the diffraction peak intensity of welded weld reduces. In addition, the irradiation hardening and the elastic modulus decreasing occur in the weld after irradiation. However, compared with the welded weld, the irradiation effect of the tempered weld is relatively weak, indicating that the anti-irradiation performance of CLAM steel weld can be improved by quenching and tempering.