A dynamic high speed laser marking system that is equipped on product lines is presented. The cause of error for marking on the fly is specifically explained in this paper. A correction method for marking on the fly is developed, in which an encoder is used for synchronization. Experiments prove that this laser marking system can effectively mark long strings under the control of the developed software. The demand of marking precision is achieved. As for the problems of marking on the fly,the available improving method is raised.