Abstract: The problem of frequency shift appears for laser as a light source under direct modulation,and the chirp characteristics of DFB laser diodes under small signal modulation are studied and analyzed.The factors and improvements on the frequency chirp are proposed,and the simulation results confirm the correctness of the analysis results.The simulation comparison between the frequency chirp of laser and IM response is made,and the similarity between the peak frequency of the frequency chirp and relaxation oscillation frequency is validated.Results show that the measures for impelling attenuation of relaxation oscillation can be used to reduce the chirp.The impacts of simple measurement of frequency chirp on the communication system are analyzed.This provides the basis for optimization design and improvement of the device performance.