In order to improve the signal-to-noise ratio(SNR) and the detection limit of the tunable diode laser absorption spectroscopy(TDLAS), the system′s second-harmonic signal and noise are analyzed.Then four kinds of digital filtering techniques are used for comparison, and four corresponding software programs written by Visual C++ language are used for noise compression and signal extraction from the original noised second-harmonic signal.Finally, the α-β-γ filter and Kalman filter are used for deno...