In order to study the effect of the active agent on the temperature of the molten pool of laser welding samples, the ANSYS 3-D finite element model of active laser welding was established with 304 stainless steel thick plate as the object. In the model, the welding temperature field was numerically simulated, and the infrared thermal imager was used to monitor the melting. Based on the comprehensive numerical calculation and experimental test data, the temperature variation trend of the specimens coated with different active agents and uncoated active agents during laser welding was compared and analyzed. The results show that the numerical simulation results are basically consistent with the experimental results. The application of the active agent does not have a significant effect on the temperature field distribution, but the peak temperature of the molten pool is slightly changed. Compared with the uncoated active agent welded specimens, SiO2 and TiO2 active agent respectively raises the peak temperature of the molten pool by about 7% to 9%, and the NaF active agent reduces the peak temperature of the molten pool by about 5%. This research will enrich and develop the basic theory of active laser welding thick plates, and provide important theoretical and experimental basis for the popularization and application of active laser welding.