The invention discloses a method for measuring length of a steel plate, relates to end cutting technology of a finishing line of a thick plate production line and solves the problem that existing steel plates are low in sectional cutting accuracy. The method includes enabling an end cutting conveying roller way to start running, enabling the steel plate to move forwards, enabling a laser velocimeter mounted at an outlet of an end cutting shear and at a position of L0 away from a cutting edge of a lower shear blade of the end cutting shear to emit laser, and transmitting feedback signals when the laser irradiates the steel plate to a PLC (programmable logic controller); enabling the PLC to start performing length measuring detection on the steel plate; comparing L with target length Lm preset in the PLC; when L is equal to Lm, enabling the conveying roller way to stop running, enabling the steel plate to stop moving, starting the end cutting shear, and automatically cutting the steel plate at a position of Lm; if the last section is not cut, cutting a next section, and continuing to execute the above steps until the last section is cut. In the method, the laser velocimeter is utilized to measure length of the steel plate, so that accuracy in sectional cutting of the steel plate is improved effectively, yield is increased, and cutting efficiency is improved while labor intensity is lowered.