The study of the dimensional optimization design method of marking device is helpful to speed up the process of self-control of the production line of marine section steel cutting. The composition and work flow of the marking device are analyzed, five design variables with the same dimension are summarized, and the design variables are normalized to four. Taken one of the design variables as the basic, the performance map based on the other three design variables is drawn. Through data comparison, the rough optimal solution of the marking device is obtained, which is substituted into the genetic algorithm as the initial solution, and the fine optimal solution of the marking device is obtained. This optimization method takes into account the characteristics of fast, intuitive and close to global optimization of the performance map method and high data accuracy of the genetic algorithm, and can obtain the global optimal solution under limited computing power. Through comparison, the efficiency of this optimization method is about 70% higher than that of direct use of genetic algorithm, which provides a theoretical basis for the domestic development of marking device.