Recently,　cloud　computing　has　emerged　as　a　new　model　of　service　provisioning,　in　this　model,　one　of　the　most　challenging　problems　is　workflow　scheduling,　i.e.,　the　problem　of　satisfying　users＇　QoS　while　minimizing　the　execution　cost　of　cloud　workflow.　This　paper　propose　a　workflow　scheduling　algorithm　called　Control　Structure　Reduction　algorithm(CSR).　In　CSR,　the　workflows　represented　by　DAG　(Directed　Acyclic　Graph)　can　be　converted　into　an　equivalent　sequence　control　structure　by　such　means　as　mergers　and　reduction.　Then　we　can　easily　identify　the　critical　path　of　the　workflow　process.　By　using　Time　Float　Distribution　Algorithm,　the　total　time　float　is　allocated　to　each　task　based　on　critical　tasks　in　critical　path,　eventually　to　enlarge　the　cost　optimization　intervals　of　all　tasks.　The　simulation　results　show　that　CSR　has　a　promising　performance　in　decreasing　the　execution　cost　for　large　workflows.