A methane leakage telemetry system based on TDLAS-WMS is proposed in this study to meet the high-precision, non-contact, and long-distance detection requirements of methane leakage. The system adopts a dual core advanced RISC machine (ARM) and field programmable gate arrays (FPGA) and design, where FPGA realizes the digital phase-locked demodulation to extract the first (1f) and second harmonic (2f) signals. Furthermore, ARM receives the 1f and 2f signals demodulated by FPGA in real time through serial peripheral interface (SPI) and relays them to Labview for harmonic signal analysis and methane concentration online demodulation. Using 2f/1f signal processing technology, the influence of change in light intensity and target reflection coefficient on the system measurement results is eliminated. The relationship between harmonic signal and detection distance in the range of 0-90 m under the same experimental conditions are studied. Results reveal that 2f/1f signal processing technology has a considerable effect on noise suppression. The harmonic signals with different integrated concentrations are measured, 2f/1f and gas integrated concentration data are linearly fitted by the least squares method, and system calibration linearity is obtained, which is 0.9966. Moreover, the system error is measured and analyzed experimentally. Within the range of 0-2000×10^-6 m, the maximum and minimum relative errors of system measurements are -3.66% and -0.23%, respectively. Using 1500×10^-6 m, the detection limit of the second harmonic signal evaluation system for methane concentration is determined to be 70.5×10^-6 m. Results indicate that the designed and developed methane leakage telemetry system can be widely used in the monitoring and early warning of sudden gas leakage in urban gas stations and natural gas pipelines.