To accurately measure the concentration of trace gas methane (CH₄) in ambient atmosphere, tunable diode laser absorption spectroscopy (TDLAS) technology was adopted, and a distributed feedback (DFB) laser with a central wavelength of 1653 nm was selected as the laser light source to build a CH₄ detection system. For the detector noise and optical interference fringe noise in the system, radio frequency (RF) noise source, multiple averaging, and Kalman filtering were added to improve the detection accuracy of the system. The experimental results show that the calibrated CH₄ concentration has an ideal linear relationship with the peak value of the second harmonic signal detected by the system by combining the long optical path multi-pass cell (MPC) and TDLAS technology. The minimum detection limit of the Kalman filtered system is 0.14 ppb when the integration time is 213 s. By determining the optimal parameters for adding RF noise sources and comparing multiple averaging techniques, a measurement accuracy of 144 ppb at an averaging time of 10 s was achieved. After applying Kalman filtering for data processing, the measurement accuracy reached 134 ppb, indicating that Kalman filtering can achieve high measurement accuracy.