A gas molecular spectroscopy measuring device with high detection sensitivity an d high stability is built by using a multi-mode diode laser near 1675nm as the light source,which can probe the CH_4transiti ons in the 2ν_3overto ne band.In order to improve the detection limit of the system,a discrete lens - type multi-pass call with path length of 100m is employed as the measurement absorption cell.The multi-mode diode laser correlation,wavelength modulation technology and long-path absorption spectroscopy are combined to imp r ove the sensitivity of the system.The CH_{4concentrations} are derived from the r elationship between the normalized WMS-2f signal peak h eights of the measurement and reference signals which are selected based on high signal to noise ratio an d correlation coefficient. The experiments are carried out at room temperature and 20.265kPa,and the CH_{4concentration} range is 1.00×10－5－1.10×10－2. The calculation results show that there is a highly linear relationship between t he measured and actual CH_{4concentrations},the linearity is 0.997over the w hole tested range,and the standard deviation is 3.50%.The stability o f 2.53% is achieved for a sample with CH4concentration of 2.60×10－4,using 30suc cessive measurements with each measurement time taking ～10s during 30min. The design can meet the needs of methan e online monitoring.The system has higher reliability and stability,and is immun e to the variation of the environment.