Optimized geometry of single-walled carbon nanotubes (SWCNTs) is vital to high-performance transparent conductive films (TCFs). Herein, the geometry of SWCNTs, i.e., tube diameter, bundle length, and bundle diameter, are successfully tuned by introducing carbon dioxide (CO2) into floating catalyst chemical vapor deposition (FC-CVD), where carbon monoxide (CO) is used as a carbon source and ferrocene as a catalyst precursor. Both tube diameter and bundle length increase with an increment of CO2 concentration, and the yield of SWCNTs can be significantly promoted with the appropriate amount of CO2. The role of CO2 in this behavior is further rationalized as enhancing CO decomposition or carbon dissolution into catalysts. The TCFs based on SWCNTs with optimized geometry by CO2 exhibit improved performance up to 86.8 Ω sq−1 at 90% transmittance after AuCl3 doping, achieving about 50% reduction of sheet resistance compared to the TCFs without CO2. The use of CO2 for directly tuning growth of SWCNTs blazes a new trail in the field of SWCNT based TCFs.