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.