A measurement system based on the spectral absorption was established to measure the N2O concentration accurately.First, the relationship of the second-harmonic, the first-harmonic with N2O gas concentration was demonstrated in theory. Then, a N2O detection system for trace concentration was designed based on a Distribute Feedback Laser(DFB).By using the light modulation and lock-in amplifier technologies, weak gas concentration signals in the strong clutter background were analyzed. Finally, the detection performance, anti-jamming capability and the repeatability of test results were verified through experiments. Test results indicate that the system can offer the measuring range from 0 to 1%, detection limit of 5.0×10-5,and the relative detection error of 0.11%. Furthermore,the linear equation is Y=192.699 09X- 0.006 24, the linearity is 0.998 07,and the relative standard deviation is 0.137%.It also proves that the CO2, O2, water vapor, et al. have no effect on the experimental results. By changing the centre wavelength of the laser,the system can be used in the detection of other greenhouse gases, such as CO2 and CH4.