The microstructure evolution of nuclear reactor pressure vessel A508-3 steel irradiated by heavy ions up to 1.5 dpa was studied by transmission electron microscopy (TEM). According to the TEM analysis, black dots were widely distributed in the irradiated A508-3 steel, with a high density of 1.782 × 10²²/m³. A large number of dislocations with Burgers vectors were formed in the irradiated A508-3 steel and tangled together, leading to the formation of dislocation networks. The number density of black dots at 1.5 dpa was 3.5 times higher than that at 0.08 dpa, and the corresponding average size showed an 8% increase. The higher density of dislocation defects led to a significant increase in hardness from 3.0 GPa at 0.08 dpa to 4.2 GPa at 1.5 dpa. The elastic modulus showed a slight increase and less dependence on the irradiation dose.