Abstract: The distribution and transport of carbon dioxide (CO₂) in the middle and upper atmosphere are closely linked to atmospheric dynamical processes, but the influence of planetary waves on CO₂ transport remains unclear. This study aims to investigate the impact of quasi-2-day waves (Q2DWs) on CO₂ transport in the stratosphere–mesosphere during the sudden stratospheric warming (SSW) period. On the basis of data assimilation (DA) using the Whole Atmosphere Community Climate Model (WACCM) + Next-generation Ensemble Data Assimilation System (NEDAS) from December 2018 to February 2019, we used the transformed Eulerian mean framework to conduct a diagnostic analysis on Q2DW propagation and amplification. Additionally, we identified unstable regions in the stratosphere between 60°N and 80°N. This configuration facilitates the amplification of Q2DWs through enhanced baroclinic and barotropic instabilities. The results demonstrate that the dynamic features exhibit distinct propagation and amplification characteristics. We also found distinct Q2DWs in CO₂ at the middle latitudes of the southern hemisphere, which were mainly induced by the Q2DWs in temperature. Further analysis showed that the anomalous vertical motion during the SSW period could lead to the enhancement of CO₂ vertical gradients, which also contributed to the strengthening of Q2DWs in CO₂.