Based on the quartz tuning fork enhanced photothermal spectroscopy (QEPTS) technology, a long-distance free space gas detection system was proposed to monitor CH4 leakage. A CH4 absorption line at 1650.961 nm was selected for this gas detection system to reduce the interference of water vapor and carbon dioxide in the atmosphere. The Allan deviation analysis showed that the integral detection limit of this long-distance free space gas detection system for CH4 detection could reach 3.79 ppb·m at the optimal integration time of 502 s. And the linear response of the proposed sensor was evaluated, with yielded R-square value of 0.997 being obtained. In addition, a CH4 cylinder with concentrations of 5% was placed in different positions to simulate the leakage of CH4 in free space, and the experimental results showed that the long-distance free space with a QEPTS gas detection system could meet the need for CH4 leakage detection in the practical engineering application.