Based　on　the　dispersion　relation　of　intense　laser　pulse　propagating　in　gradually　ionized　plasma,　this　paper　discusses　the　frequency　modulation　induced　by　ionization　of　an　ultra-short　intense　laser　pulse　interacting　with　a　gas　target.　The　relationship　between　the　frequency　modulation　and　the　ionization　rate,　the　plasmas　frequency　variation,　and　the　polarization　of　atoms　(　ions)　is　analysed.　The　numerical　results　indicate　that,　at　high　frequency,　the　polarization　of　atoms　(　ions)　plays　a　more　important　role　than　plasma　frequency　variation　in　modulating　the　laser　frequency,　and　the　laser　frequency　variation　is　different　at　different　positions　of　the　laser　pulse.