The invention relates to a method for penetrating a straightening steel plate of a thermal straightening machine, belonging to the technical field of thermal straightening machines for plates of moderate thickness. The method comprises the following steps of: when a head part of a steel plate enters the inlet of a straightening machine, setting a roll gap set value of the straightening machine to be M=H+X; when the straightening machine is operated in a straightening conveyor belt mode and the head part of the steel plate of a first pass reaches the outlet of the straightening machine, enabling the roll gap to decrease from M, and when the roll gap value of the straightening machine is reduced to the steel plate thickness value H, beginning to straighten, wherein a low-pressure straightening mode is used for penetrating and straightening of the straightening machine, the straightening pressure is 1,000kN-6,000kN, and the system pressure for pressing a hydraulic cylinder of the system is controlled to be between 200bar-250bar; the steel plate enters the straightening machine area at a high speed and bites into the straightening machine at a low speed, and the speed in a steel plate straightening process is high so as to achieve appropriate straightening efficiency. The invention has the advantages that the method effectively reduces percussion of the head part of a steel plate which has a length of 15 meters and a raised head to a straightening roller.