“In the field of research on vacuum failure accidents in fusion reactors, experts have established atmospheric diffusion models and analyzed the impact of tritium diffusion distribution, providing solutions for accident consequence evaluation. In the event of a vacuum loss accident in a fusion reactor, tritium will leak into the environment. Simulating tritium atmospheric diffusion is an important aspect of evaluating the consequences of fusion reactor accidents. Based on the Gaussian plume model and Pasquill stability classification method, considering the influence of factors such as gravity settling, smoke lifting, and wind speed, an atmospheric diffusion model suitable for transient analysis under accidents was established. The ground reflection coefficient, which corrected the image source contribution, was added to the Gaussian plume model to improve the calculation effect of the model on dry settlement at the ground boundary. The accuracy of the established model was verified by selecting the tritium release experiment in Canada and the tritium release accident at the Savannah River plant in the United States. The calculation results of the model were comparable to the accuracy of HotSpot 3.0 and UFOTRI software. The vacuum loss accident of the International Thermonuclear Experimental Reactor (ITER) was selected as the research object, and the effects of tritium release in stages, wind speed, and release height on tritium diffusion distribution were analyzed. The results indicate that the staged release of tritium will lead to the appearance of two highly radioactive areas along the downwind direction; The increase in release altitude and wind speed will enhance the diffusion behavior of tritium in the atmosphere, thereby reducing the accumulation of radioactivity in the near-field.”