Abstract: Effects of different tempering temperatures on the microstructure and properties of 80 kg direct quenched steel were studied. The results show that the direct quenched steel has high strength and toughness combination and high temperature tempering resistance. When the tempering temperature is 450-650℃, the test steel still maintains the shape and orientation of the lath martensite.With the increase of tempering temperature, the slab coarsening occurs, accompanied by the decomposition of retained austenite and the precipitation and growth of carbide. At the tempering temperature of 700 ℃, a high degree of recrystallization occurs in the test steel, and the properties deteriorate obviously.When the tempering temperature is 650 ℃, the test steel has the best comprehensive mechanical properties, the yield strength is 795 MPa, the tensile strength is 821 MPa, the elongation after break is 20.6%, and the impact work of the standard sample at-20 ℃is 195 J.