The invention relates to a flame cutting method of a thick steel plate. The flame cutting method comprises the following steps: three flame guns are mounted on a machine head of a flame cutting machine for igniting secondary flame of a flame gun I, a flame gun II and a flame gun III in sequence; then, main flame of the flame gun III is ignited, and the flame is adjusted; the machine head of the cutting machine is shifted, so that the flame gun I is positioned on a cutting path; the flame gun II is positioned in a cutting preparation starting position; main flame of the flame gun III can heat the lower half part of a metal plate to be cut; when an ignition temperature is reached, a cutting oxygen valve is opened; and when the flame guns reach the terminal of the metal plate to be cut, the flame guns are closed. The flame cutting method proposes a combined flame cutting principle, that is, the system combination of the three flame guns and corresponding functions are used for realizing the cutting of the large-thickness metal plate. The flame cutting method realizes smooth cutting of workpieces with ultralarge thickness, and improves the economy.