Quantitative remote sensing of natural gas pipeline leakage based on laser absorption spectroscopy is reported. Key technical issues of mobile laser remote sensing are discussed through simulating leak detection. For quantitative remote sensing of trace leakage, the signals with counter-phase and isovalue of residual amplitude modulation are introduced. Then the offset is compensated, the influence of harmonic signal is reduced and sensitivity is improved. The improved soft-threshold denoising method is proposed by analyzing the characteristics of echo absorption spectrum. The results show that the signal-to-noise ratio is improved two times as great as that by the traditional soft-threshold denoising method and the shape of second harmonic (2f) signal is well preserved. The minimum remote sensitivity will be 80 ppm/m under the mobile remote sensing.