Existing　speech　enhancement　methods　can　improve　speech　quality　but　not　speech　intelligibility,　especially　in　low　SNR　conditions.　To　solve　this　problem,　an　algorithm　for　improving　speech　intelligibility　using　the　Constraints　on　Speech　Distortion　and　Noise　Over-estimation　(CSDNO)　is　proposed　in　this　paper.　Based　on　the　fact　that　the　attenuation　distortion　and　amplification　distortion　have　different　impacts　on　the　speech　intelligibility,　the　gain　function　and　noise　estimation　method　used　in　conventional　algorithm　are　modified　in　this　paper.　The　performance　of　the　proposed　method　has　been　evaluated　by　Diagnosis　Rhyme　Test　(DRT),　fractional　Articulation　Index　(fAI)　and　frequency-weighted　SNR　segmental　(fwSNRseg)　for　three　types　of　noises.　The　experimental　results　show　that,　with　smaller　speech　distortion,　the　proposed　method　can　improve　the　intelligibility　of　the　enhanced　speech　in　comparison　with　the　reference　method.