The　mechanism　of　the　modification　of　Kevlar　fiber　by　polypropylene　glycol　(PPG)　and　cis-2-butene-1,4-diol　was　studied　in　the　paper,　the　authors　learned　the　esterification　of　toluene-2,4-diisocyanate　(TDI)　onto　Kevlar　fiber　by　infrared　spectrum.　In　the　mean　time,　the　infrared　spectrograms　of　the　productions　which　steadily　disposed　by　PPG　and　butendiol　were　analysed　respectively,　the　result　showed　that　the　intensity　of　the　bands　was　reinforced　at　about　1700-1720　cm-1　after　the　samples　were　steadily　disposed,　that　is　to　say,　the　group　of　-NCO　has　been　stabilized　into　-NHCO　group,　the　effect　of　steady　disposal　was　obvious;　but　the　disposal　effect　of　butendiol　was　apparently　better　than　PPG＇s　at　the　same　condition.　Finally,　the　authors　compared　the　influence　of　different　mol　rates　between　TDI　and　butendiol　on　the　productions.　Based　on　the　consequence,　excessive　butendiol　would　prevent　the　Kevlar　fiber　from　farther　reaction,　therefore,　the　mol　rate　between　TDI　and　butendiol　should　approach　1:1.