Hydrogen Sulfide is a highly toxic and flammable gas. It is generated as a common by-product of many industrial processes. Early detection of H2S at concentrations of 10 ppm in air is essential to prevent its toxic influence at higher concentrations. Tunable diode Laser based gas detectors are now being used in a wide variety of applications for safety and environmental interest. Their characteristics like non-intrusive, high selectivity, high sensitivity, and fast response time make them more appealing than conventional point sensors for a reliable H2S detection. A long open-path H2S sensor based on tunable diode laser absorption spectroscopy has been developed, using telecommunication near infrared distributed feed-back(DFB) tunable diode laser combining with wavelength modulation spectroscopy technology, a detection limit of 20 ppmldrm has been demonstrated, an on-board reference cell provides on-line sensor calibration. The sensor is very suitable for large area field H2S monitoring application.