In order to increase the strength of D6AC steel to the ultra-high strength steel level with adequate ductility and toughness，orthonormal testing method was used. Only nine experiments were made and the materials expended in this method are less than that of conventional method. Moreover，in a short time the better technological condition of I.H.T(Intercritical Heat Treatment)was found out. Prior-heat treatment (Ouenching and high tempering)+795℃ quenching+low tempering. The values of the mechanical properties which would be attained possibly were pre-estimated under such conditions. Results show that the difference between the values of pre-estimated values and those measured in the experiment is very litt1e.