Reduced activation ferritic/martensitic steels are considered as one of the most suitable materials in the application of international thermonuclear experimental reactor.One of the important requirements for the applications is to retain proper mechanical properties in irradiation and high temperature conditions.In this work,a multi-scale model,which consisted of models in different scales and theories for different strength methods,was established to simulate the yield strength and stress-strain curve of steels in both irradiation and high temperature condition.Simulation results showed that,at room temperature,P-N stress and grain refinement occupied relatively large proportion.With the increase of temperature,the great decrease of shear modulus,lattice resistance and grain boundary strength led to a great weaken of P-N and grain refinement stress.The simulation results of yield strength were basically consistent with experimental results.