A seismic activity monitoring instrument is required in gas geochemical exploration (GGE) to measure methane (CH4) and carbon dioxide (CO2) near the geological fault zone. A prototype of CH4 and CO2 sensing system based on wavelength modulation spectroscopy, which can achieve a high-accuracy, wide-range, and in situ measurement, is developed. An environment-adaptive temperature control system, which can ensure temperature stability and reduce power consumption by changing the target temperature according to the environment temperature, is proposed to adopt the sensing system to in situ environments. A state parameters correction (SPC) method based on black box theory is proposed to improve measurement accuracy, which is used to correct gas temperature and pressure, effective optical path length, initial light intensity, and constant associated with the system. Gas temperature and pressure are corrected according to the difference between target temperature and calibration temperature. The effective optical path length is corrected using target temperature and initial light intensity. Constants associated with the system are measured using the same gas at different target temperatures. A combination of the four absorption lines at 4979.48, 4979.78, 4293.53, and 4293.59 cm−1 combined with time division multiplexing (TDM) is utilized to realize wide-range measurement. The measurement accuracy of the sensing system is better than 1% in the environment temperature range from 0 °C to 35 °C. The optimal minimum detection limits (MDLs) are 0.087 ppmv for CH4 and 7.8 ppmv for CO2 when the integration times are 225 and 51 s, respectively. A test is performed in the cave in the center of Fengman Seismic Station using the sensing system to monitor CH4 and CO2. The maximum deviations of three measurements for the same gas are less than 3.9 ‰ for CH4 and 8 ‰ for CO2. The measured concentration ranges are from 2475 to 2950 ppmv for CO2 and from 135 to 137 ppmv for CH4, validating ...