Tunable diode laser absorption spectroscopy (TDLAS) is a detective technology with high sensitivity,high resolution and rapid response ,which has been used abroad for the measurement of trace gas in atmosphere and the diagnosis of toxic and harmful gas in the industry process,and the detection of nature gas leakage. Due to the features of tunability and narrow linewidth,the distributed feedback (DFB) diode laser can tune the laser output wavelength to a single absorption line of the gas precisely,which makes the TDLAS technique can be utilized to measure gas concentration with high sensitivity. A brief description of on-line gas monitors using wavelength modulation spectroscopy with second-harmonic (WMS-SH) detection is presented. A discussion of feasibility of the gas detection based on the least squares method is also presented. After the application of the modification-weighted moving average filter,the system response time is not more than 1 s. Both the signal to noise ratio and the detection sensitivity have been improved. The detective approach has been applied in the real-time detection of hydrogen sulfide in natural gas treatment factory.