As a highly toxic gas, CO is one of the culprits of air pollution. Long-term inhalation will also cause great harm to the human body and even death. How to achieve rapid CO monitoring is an important challenge in sensing field. CO as one of the of air pollution, long time inhalation will reduce the oxygen carrying capacity of blood and in severe cases death. Therefore, effective monitoring of CO is necessary. In this study, porous CeO₂ nanosheets (CeO₂ NSs) were obtained via flame annealing intermediate product CeCO₃OH nanosheets synthesized by simple hydrothermal method. Through controlling of the flame time, oxygen vacancies were introduced into the CeO₂ NSs. As a result, the CeO₂ NSs annealed with 2 min (CeO₂-2min NSs) exhibited outstanding reproducibility and selectivity towards CO gas. Particularly, the response/recovery time were extremely fast (2 s/2 s) towards 500 μL/L CO at 450 ℃ as well as finely functional relationship between response and concentration of CO at a wide detection range (10-10000 μL/L). The superior gas sensing performance of CeO₂-2min NSs can be ascribed to the high aspect ratios of the porous two-dimensional structure and abundant oxygen vacancies in the crystals. This work may supply a strategy for designing ultrafast gas sensors which can detect target gas at a wide range.