Online monitoring of end-tidal carbon dioxide (EtCO2) concentration held substantial clinical diagnostic value, as it provided insight into a patient’s respiratory and metabolic status. The wavelength modulation spectroscopy (WMS) method, due to real-time capability, high precision, and excellent gas selectivity, was widely used for the measurement of end-tidal carbon dioxide (EtCO2) concentration. The Beer-Lambert law was approximated using a first-order Taylor series in traditional WMS methods, resulting in a strong linear relationship between gas concentration and the second harmonic amplitude. However, the measuring errors increased with higher gas sample concentrations, particularly when the concentrations exceeded 10%. Therefore, in this study, an optimized WMS method was proposed, which was innovative in that it adjusted the phase of the lock-in signal to account for the phase shift caused by laser linear modulation. This approach eliminated the reliance on the first-order Taylor series approximation of the Beer-Lambert law and mitigated the influence of the laser linear modulation coefficient. Subsequently, a series of CO2 concentration gradients (≤ 20%) were used for detection experiments employing the second harmonic method, the 2f/1f method, and the optimized WMS method. The experimental results demonstrated a clear linear relationship between gas concentration and second harmonic amplitude. For the lower gas concentration range (1% to 5%), the 2f/1f method demonstrated the highest measurement accuracy, with errors less than 0.1%, while the optimized WMS method also performed well, with errors not exceeding 0.2%. However, for higher concentrations (5% to 20%), the optimized WMS method exhibited significantly smaller errors and remained stable at around 0.2%, while the errors of the other methods increased substantially. Therefore, the optimized WMS method achieved high measurement accuracy even in high-absorbance end-tidal carbon dioxide (EtCO2) measurements, highlighting its superiority in wide-range gas concentration detection.