Now　there　are　trillions　of　web　applications　and　the　number　is　still　increasing　quickly.　These　web　applications　contain　large　amounts　of　data　and　functions　which　are　useful.　However,　the　data　and　functions　cannot　be　shared　and　reused　because　these　web　applications　do　not　provide　proper　APIs.　We　can　encapsulate　the　data　and　function　API　through　recording　Web　browser　events　and　replaying　these　operations　case　by　case,　but　the　difficulty　and　workload　of　behavior　simulation　is　high.　This　paper　proposes　a　scenario　driven　approach　to　API　generation　for　B/S　web　application,　which　can　be　used　to　encapsulate　the　data/function　APIs　of　different　web　applications　semi-automatically.　First,　we　introduce　the　formal　definition　of　API　generation.　Second,　we　illustrate　the　generation　algorithm.　The　experiment　on　a　real　world　web　application　demonstrate　the　feasibility,　effectiveness　and　benefits　of　our　approach.