Airbone optical detection system for monitoring the methane gas leak is developed. The measuring principle, concentration equation, measurement device, airborne clamping apparatus and tracking pointing control system is studied.Firstly, the absorbing and measuring mechanism is designed based on the principle of near-infrared tunable diode laser absorption spectrum, using a second derivative wavelength modulation technique, the gas concentration is deduced. Then, the detailed description of measurement device, airborne clamping apparatus and control system is given. The device is mounted on low-moving platform, simulation of the gas line leak experiment shows that this method has high accuracy to tracking and pointing the detected gas line, the detection sensitivity of the system for the buried pipeline leakage is about 50 ppm*m (in the case of methane) and the tracking and pointing accuracy of the detection laser for the pipeline is 1°. This system can be applied into the airborne remote sensing detection for the long-distance natural gas leak.