The deuterium-tritium fusion reaction is the fastest commercially achievable artificially controlled fusion reaction. However, tritium, as a fuel and neutron radiation-activated material, poses radioactive safety challenges. To simulate the migration of tritium and radioactive dust in the environment under accident conditions, a dispersion simulation program called ACCTRI (ACCidental model for TRItium release) has been developed based on the modified Gaussian multi-puff model, considering dry and wet deposition and tritium re-emission effects. The concentration and dose results calculated using ACCTRI are very close to those calculated using UFOTRI and HotSpot, with a maximum difference of no more than one order of magnitude. A comparison with the experimental results reveals that the results of ACCTRI with tritium buoyancy correction are close to the experimental data. ACCTRI has good accuracy and can be used as a reference for environmental radioactivity safety in fusion reactor site selection and hypothetical accident analysis.