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Methane (CH₄) is the main pollutant in oil and gas production. The detection and accounting of CH₄ is an important issue in the process of greenhouse gas control and emission reduction in oil and gas industry. In this study, a portable CH₄ detector based on tunable diode laser absorption spectroscopy (TDLAS) technology was deployed. The three-dimensional distribution of CH₄ in a natural gas purification plant in Sichuan was obtained through vertical unmanned aerial vehicle (UAV) flight observations and ground mobile observations. According to the mass balance method, the emission of CH₄ on 30 m above ground level (AGL) and 60 m AGL in this site was about 0.012 kg/s (±42% at 1σ) and 0.034 kg/s (±47% at 1σ), respectively, in one day. The vertical distribution showed that the CH₄ concentration reached the maximum (2.75 ± 0.19 ppm) with height of 0 to 100 m AGL. The CH₄ concentration from 100 to 300 m AGL showed a downward trend with height. Atmospheric instability at high altitude and high wind speed promoted the diffusion of CH₄. The CH₄ concentrations of horizontal distribution on 30 m AGL and 60 m AGL were 2.48 ± 0.11 ppm and 2.76 ± 0.34 ppm. In the observation of mobile campaigns, the connecting equipment of natural gas treatment facilities was prone to leakage, such as in valves and flanges. CH₄ leakage was also detected at the torch mouth, especially when there was an open flame at the torch mouth. During the mobile movement investigation, the downwind measurement (OTM-33A) was applied to determine the overall CH₄ emission rate shortly after patrolling the site. This work plays a vital role in optimizing the operation and maintenance of natural gas production stations pipe network, ensuring human safety and minimizing greenhouse gas emissions.