The 16MND5 RPV steel by electrical additive manufacturing was aged at 450 ℃ for 3 000 h. The effect of aging time on mechanical properties and structure was analyzed by mechanical properties test and microstructure observation. The experimental results show that with the extension of aging time,the tensile strength and yield strength of the specimens were increased by 5 MPa and 6 MPa,respectively,and the elongation and fracture shrinkage after the fracture remained unchanged. The stable crack propagation energy W,p1, of the sample decreases,resulting in a decrease in crack propagation energy W,p,resulting in a decrease in total impact energy W,t, of 47 J. The microstructure is still pro-eutectoid ferrite and granular bainite before and after long-term thermal aging.