The invention provides an online laser marking device for a steel plate. The online laser marking device comprises a support truss, a horizontal moving device, a vertical moving device, a laser follow-up system, a speed sensor, a control and operation system, and a position sensor, wherein as the movement distances of the horizontal moving device and the vertical moving device are relatively long,rack-pinion mechanisms can be selected and used, and motors can use servo motors or step-servo motors, so as to achieve precise control easily; the horizontal moving device moves along with the steelplate after the laser follow-up system is in place; because the distance from a laser outlet to the steel plate affects the marking quality, the vertical moving device is used to precisely control the distance from the laser outlet to the steel plate, so as to ensure the marking quality; and the laser outlet of the laser follow-up system is provided with a precise distance sensor which performs closed-loop control together with a Y-axis servo motor, so as to ensure that the distance from the laser outlet to the steel plate meets a requirement. The online laser marking device achieves the online laser marking of the steel plate. The device has the characteristics of high sensitivity, high degree of automation and simple operation, and thus is easy to promote for use.