Based on the technology of tunable diode laser absorption spectroscopy (TDLAS) in conjunction with second harmonic wave detection,a long open-path TDLAS system using a 1.65 μm InGaAsP distributed feedback laser was developed,which is used for detecting pipeline leakage.In this system, a high cost performance Fresnel lens is used as the receiving optical system,which receives the laser-beam reflected by a solid corner cube reflector,and focuses the receiving laser-beam to the InGaAs detector.At the same time,the influences of the concentration to the fluctuation of light intensity were taken into account in the process of measurement,and were eliminated by the method of normalized light intensity.As a result,the measurement error caused by the fluctuation of light intensity was made less than 1%.The experiment of natural gas leakage detection was simulated,and the detection sensitivity is 0.1×10-6 (ratio by volume) with a total path of 320 m.According to the receiving light efficiency of the optical system and the detectable minimum light intensity of the detector,the detectable maximal optical path of the system was counted to be 2 000 m.The results of experiment show that it is a feasible design to use the Fresnel lens as the receiving optical system and can satisfy the demand of the leakage detection of natural gas.