With the rapid development of gas observation technology in seismic fracture zone, in order to accurately predict the earthquake, and reduce the people’s lives and property losses caused by earthquake, a mid-infrared methane sensor was designed and developed, which is based on the microscopic relation between methane release and earthquake fissures on the crustal rocks. This instrument utilizes quantum cascaded laser (QCL) operating at 7.65 μm, combined with MIR multipass herriott cell with 76 m absorption path length to obtain low detection sensitivity down to 40 nmol·mol-1 level in 4s acquisition time. Meanwhile, to decrease the primary noise source (1/f noise), semiconductor laser frequency modulation of direct absorption technology was utilized to obtain gas detection limitation as low as 5 nmol·mol-1 (40 s acquiring time). In field experiments, controllable vibrator was used as vibration source, a number of trace methane detectors were placed with 100 m distance interval to carry out the dynamic measurement of methane concentration on the ground surface at different distances from the vibration source. Experimental results show that the instrument can monitor the release of underground methane before the earthquake and provide a novel measure as a pre-alarming for earthquake.