In order to reduce the effect of electric circuit and optical path noise on the trace gas detection and achieve the purpose that further improving the accuracy of trace gas detection system, ultra-narrow-bandwidth laser is utilized as the system light source, and harmonic detection and improved Hilbert-Huang transform (IHHT) filtering algorithm are applied as the key algorithms of the data processing to construct the high precision trace gas detection system. Methane is used as the detect object, and theoretical calculation result shows that when the output center wavelength of the system light is 1650.959 nm, the correlation coefficient between methane gas volume concentration C and the first, second harmonic generation ratio of the output intensity I2f/If is 0.084639. The sensitivity experiment is carried out by inject specific fraction volume methane into the platform optical gas cell. The experimental data show that after the application of IHHT to reduce noise, the correlation coefficient between methane C and I2f/If is upgraded from 0.062585 which is obtained by applying fast Fourier transform (FFT) to process the data to 0.074884. The better effect of improved IHHT noise reduction depends on the less methane gas volume fraction.