Abstract: Porosity is the main drawback during welding of galvanized steel sheets. In order to investigate the escape process of the zinc vapor produced during laser welding of galvanized high-strength steel plate (DP780), a high speed camera was used to record this escape process and channels. Based on the observation, the formation mechanism of internal porosity and external porosity was summarized, and the two-channel dynamic balance control method of zinc vapor escape was summarized based on this mechanism. By observing the test process and analyzing the actual formation rate of porosity, it is proved that the dynamic balance of the two escape channels can reduce the porosity of the welded joint. The experimental results show that, by improving the dynamic equilibrium of escape route of zinc vapor, the porosity of welded joints decreases 3.99 times. It shows that this method can well explain the splashing and external porosity defects of galvanized high strength steel plate, and improve the welding process parameters, so as to improve the efficiency of zinc vapor escape and the welding quality and appearance of galvanized high strength steel plate.