Abstract: Aiming at the Hunga Tonga–Hunga Ha"apai volcanic eruption in January 2022, a more reasonable design of volcanic ash source term was established by comprehensively adopting empirical statistical formulas, segmented particle size spectra, and umbrella-shaped vertical mass distribution models. On this basis, the Lagrangian particle dispersion model (FLEXPART) was employed to investigate the long-timescale global transport and diffusion characteristics of volcanic ash. The results show that: (1) According to preliminary estimation, the total volcanic ash masses of the two eruptions on 13 January and 15 January were 3.36×10^11 kg and 1.81×10^13 kg, respectively. (2) Volcanic ash predominantly transported westward within the latitudinal zone of 0–30oS at a speed of approximately 24o per day, with weak north-south dispersion. The transport altitude ranged from 18 to 32 km, and the fastest transport layer was concentrated at 25–27 km. Vertical wind speed differences were the primary factor leading to the inclined vertical transport of volcanic ash. (3) The particle size of volcanic ash presented a strong correlation with its atmospheric residence time. Volcanic ash particles with diameters greater than 62.5 μm and those ranging from 7.8125 μm to 62.5 μm completely deposited to the ground within several hours and half a month (before 20 January), respectively. The major deposition region was located at (175oE–170oW, 5oS–50oS), with the maximum surface deposition flux exceeding 1000 μg m^-2. Fine volcanic ash with diameters smaller than 7.8125 μm remained suspended in the atmosphere for an extended period with slow deposition progress.