When using the variational mode decomposition and wavelet threshold adjustment function（VMD-WTFD） algorithm to deal with noise, the signal waveform recovery often deviates from the standard harmonic signal, resulting in deviations of the inversion results. An improved VMD-WTFD algorithm is proposed, which uses a new threshold function to improve the VMD-WTFD algorithm, and then uses this improved algorithm to perform inversion analysis on the second harmonic spectral signal. The simulation and experimental results show that compared with several other denoising algorithms, the improved VMD-WTFD algorithm can invert the peak, peak position, and peak valley values of the original signal under different modulation degrees more closely to the original standard second harmonic spectral signal, with a signal-to-noise ratio of 17.545 dB and a correlation coefficient of 0.992 9 after denoising. It can clearly invert the second harmonic amplitude of CO2 in human exhaled gas.