Abstract: Attention was paid to four factors of heat treatment, such as, quenching temperature, medium water concentration, cooling time and tempering temperature. In order to find out the optimum heat treatment, comprehensive mechanical properties were used as evaluating index, and orthogonal test was designed. The results show that quenching temperature of 1 060 ℃, AQ251 medium water concentration of 12%, cooling time of 3.5 min and tempering temperature of 640 ℃ were the best match. After the optimum treatment, the tensile strength was 974 MPa, the yield strength was 882 MPa, the elongation was 16.4%, the reduction of area was 52.6%, the impact energy was 18 J, the hardness was 315 HB, the microstructure was homogeneous and grain size reached 7.5 grade.