Quantifying the interregional transport of atmospheric carbon dioxide (CO2) between Indo-China Peninsula and Chinese mainland is of significant importance for the carbon budget accounting. However, few studies have addressed the mutual impacts of atmospheric CO2 between the two regions. In this study, a regional atmospheric transport model (WRF-RAQMS) coupled CO2 physical processes was applied to investigate the seasonal variation of atmospheric CO2 concentrations over the Indo-China Peninsula and Chinese mainland, and to reveal the mutual impacts between the two regions by taking the year 2020, when COVID-19 occurred, as an exemplification. The simulated atmospheric CO2 concentrations in the study domain exhibited distinct seasonal variation, and higher concentrations occurred in spring and winter, with regional average exceeding 413.0 ppm, and the lowest concentration appeared in autumn, which were generally consistent with results from multi-source validation datasets. The daily variations and three-dimensional spatial distribution of net changes in atmospheric CO2 concentrations across the four seasons were also reproduced. The impacts of the anthropogenic activities and natural biomass burning in Indo-China Peninsula on the tropospheric CO2 concentrations over Chinese mainland decreased gradually from spring to winter, at 24.13%, 5.02%, 3.64%, and 0.92%, respectively, with the majority contributions originated from biomass burning. Conversely, the most significant impacts of the anthropogenic activities and natural biomass burning in Chinese mainland on the tropospheric CO2 concentrations over Indo-China Peninsula were occurred in winter (13.35%) and almost all CO2 transported from Chinese mainland to the Indo-China Peninsula originated from anthropogenic emissions.