Underground coal gasification is an important way to realize clean and efficient utilization of coal. In order to realize the detection of methane produced by underground coal gasification, a wide-spectrum measurement technique based on tunable diode laser absorption spectroscopy (TDLAS) for methane at high temperature and high pressure and the corresponding gas parameter inversion method are proposed in this work. Firstly, a wide-spectrum measurement system for high temperature and high pressure methane was established based on a tunable semiconductor laser with a wide wavelength tuning range. Then, the absorption spectrum of 2.05% standard methane gas was measured using the system at temperatures of 200 ^oC and 400 ^oC and pressures of 0.5 × 10⁶–3.0 × 10⁶ Pa, with spectral wavenumber ranging from 6010–6190 cm^-1. Finally, based on absorption spectroscopy, simultaneous inversion of multiple parameters including methane concentration, temperature and pressure were achieved using the least squares fitting algorithm, and the inversion errors of each parameter were all within 10%. The research provides an effective and feasible means for on-line detection of methane gas produced by underground coal gasification.