The invention discloses a preparation method of bainite / martensite / austenite composite high-strength steel, belonging to the field of low-alloy high-strength steel. According to the composite high-strength steel, C, Mn, Si and Cr are used as main alloy elements, Al, Ni, Mo, Cu, Nb, Ti, V and the like are added, and the balance is Fe. The preparation method comprises the following steps: smelting, casting, forging or rolling to prepare rails, thick-wall pipes, large-section bars or thick pipes and the like by adopting a conventional steel-smelting process; carrying out austenitizing; controlling cooling by adopting different cooling media to obtain a part of bainite tissue in the cooling process; instantly carrying out distribution treatment and low-temperature tempering treatment so as to finally obtain a bainite / martensite / austenite composite tissue which is good in strong plasticity and toughness coordination, wherein the distribution time is 30-360 minutes. For a component which is large in size and easy to crack in quenching, the bainite / martensite / austenite composite high-strength steel has wide application prospect.