The invention relates to an ultrahigh-strength high-toughness multistep isothermal bainite steel and a preparation method thereof. The technical scheme of the invention is as follows: a hot-rolled steel billet is austenitized 0.2-5.0 hours at a temperature of 850-1100 DEG C firstly, and then the steel billet is subjected to bainite multistep isothermal transformation, so that an ultrahigh-strength high-toughness multistep isothermal bainite steel is obtained. The multistep isothermal transformation is implemented through the following steps: 1, carrying out heat preservation 0.2-3.0 hours in an isothermal medium at a temperature of 250-450 DEG C; 2, carrying out heat preservation 2-50 hours in the isothermal medium at a temperature of 220-350 DEG C; 3, carrying out heat preservation 24-240 hours in the isothermal medium at a temperature of 180-250 DEG C; and 4, carrying out heat preservation 50-360 hours in the isothermal medium at a temperature of 150-220 DEG C, and cooling the obtained product to room temperature, wherein the multistep refers to step 1 to step 2, or step 1 to step 3, or step 1 to step 4. The bainite steel prepared by using the method disclosed by the invention has the characteristics of high hardness, high strength, high toughness, good volume stability and fine microstructure.