End-tidal carbon dioxide (P(ET)CO₂) monitoring has become an important tool in clinical monitoring, but there are still limitations in practice. Low-frequency modulation was used to reliably acquire respiratory information. Then the disturbances of humidity and flow rate were removed by signal decomposition. Finally, the real-time concentration of CO₂ was calculated and displayed by an adjusted calibration function. Targeted experiments confirm that a period of 180 ms and a depth of 50% was the optimal choice. In this case, the effects of humidity and flow rate reflected by different components were removed effectively from the capnography. This capnometer obtains capnography with excellent accuracy and stability in long-term continuous monitoring.