We present a novel method for analyzing the nonlinearity of the DFB laser for the first time.The method is based on a spectrum analysis with discrete Fourier transform（DFT）on the model of laser mapping functions. In view of the signal analysis

it avoids using some semiconductor parameters that are difficult to measure but necessary for analyzing in the method. The algorithm together with the mapping function derived from the measurement can examine the harmonic and intermodulation distortions of laser chip and laser property of DFB laser module. The method is also applied to simulate the linear property of DFB laser and evaluate the linearity request for the laser chips. The comparison between the measurement and calculation shows that the algorithm is simple and effective for laser distortion analysis.