By combining numerical simulation and experimental investigation, this paper intensively investigated the influence rules of adding SiO2 active agent on the dynamic behaviors of flow field in the laser welding 304 stainless steel thick plate pool. First of all, the mathematical model of the activating laser welding stainless steel thick plate was constructed to numerically investigate the impact of active agents on the fluid flow in the pool. Secondly, based on the tungsten particle tracing principle test, the high speed camera system was adopted for real-time monitoring to investigate the mechanism of action of the active agents on the flow field in the pool. The results suggested that, the active agent partially affected the mode of fluid flow. The velocity vector from both sides of the pool to the center occurred on the pool surface, while vortex from the pool margin to the center occurred on the pool cross section. It was suggested from the phenomenon that the tracing tungsten particles moving from the pool margin to the center that, it shows that the flow direction of the fluid changed after coating with the active agent, the fluid flew from the pool margin to the center. At the same time, the coating of active agents changed the flow velocity of fluid inside the pool, the maximum velocity of fluid flow on the upper surface, the vertical section and the cross section at the same position of the active agent coated specimen increased by 25.8%, 8% and 33.8%, respectively, compared with that without the active agents.