Aiming at the problems of complicated background gas composition， difficult detection， and poor stability of methane in the atmosphere， based on tunable diode laser absorption spectroscopy technology and wavelength modulation spectroscopy technology， the cascaded integrator comb filter and finite impulse response filter are applied to the digital quadrature lock-in amplifier to investigate experimental research on trace detection of methane gas in the atmosphere in this paper. Experiments show that compared with the traditional digital quadrature lock-in amplifier， the signal-noise ratio of the second harmonic signal extracted by the improved digital quadrature lock-in amplifier is improved from 38.61 dB to 44.95 dB； Applying the nonlinear iteration partial least square-extreme learning machine algorithm model to the methane gas concentration inversion， compared with the classic least square method， the root mean square error decreases by 0.907. Through 16 sets of concentration step-by-step experiment tests， the actual detection limit of the system is 1 ppm； The long-term stability test is conducted at 600 mbar pressure， 25 ℃ and 50 ppm methane concentration for 3 hours. The variation range of methane concentration detected is 49.6 ppm~50.3 ppm， and the standard deviation is 0.092 1 ppm. When the integration time reaches 56 s， the theoretical detection limit of the system is 25.6 ppb. This shows that the cascaded integrator comb filter and the nonlinear iteration partial least square-extreme learning machine algorithm model have higher superiority and practical prospects in infrared gas detection.