Abstract: 8418 steel is an air-cooled hardening hot work die steel containing Cr, Mo, V and other elements, which is widely used in hot forming fields such as hot forging dies, hot heading dies, punching dies, die-casting dies and extrusion dies. In this paper, the influence of heat treatment on the microstructure and mechanical properties of forged 8418 die steel was studied by using metallographic microscope, hardness test, tensile test and impact test to study the 8418 steel produced by a domestic casting and forging company. The results show that the grain size grade of 8418 steel is increased from the original 6.5 to 8.5 after heat treatment (ultra-fine treatment and spheroidization annealing, 880°C normalizing, 1030°C quenching and two 600°C tempering), and the final structure is a tempered sorbite with fine grains. The strength, hardness, plasticity, toughness and impact properties of the material have been significantly improved, and the yield strength of the material after final heat treatment of 8418 steel is 1660MPa, the tensile strength is 1800MPa, the elongation is 17.5%, the hardness is 564.7HV, and the impact absorption energy is > 300J, showing excellent comprehensive mechanical properties.