Abstract: Future advanced nuclear energy systems such as fusion reactors require materials to maintain good structural stability and mechanical properties for a long time under high-flux high-energy neutron irradiation. In order to satisfy the requirements of the development of future advanced nuclear energy technology, the China Low Activation Martensitic (CLAM) steel was developed by the FDS Team, Institute of Nuclear Energy Safety Technology, Chinese Academy of Sciences, under wide domestic and overseas collaborations. In the design of CLAM steel, following features were considered according to the property requirements such as low activation, intense irradiation resistance, and high temperature and corrosion resistance of the service environments in advanced nuclear energy. The low activation compositions were designed with neutron activation analysis, while neutron irradiation and heat resistance properties were designed and optimized by a selective nano-precipitation mechanism. To evaluate the irradiation performance of CLAM steel, a series of irradiation tests were carried out by using neutron, ion, electron and plasma irradiation facilities both in China and overseas. The microstructures and macroscopic properties before and after irradiation were multi-perspective analyzed. The irradiation resistance properties of CLAM steel were compared with those of the similar materials developed in other countries. The results show that CLAM steel has good irradiation resistance.