The　research　in　this　paper　presents　investigation　on　the　load　bearing　capacity　of　fame　composite　slim　beam　with　deep　deck　and　proposes　one　calculation　method　of　flexural　stiffness.　Based　on　the　classical　Euler-Bernoulli　theory,　a　formula　to　calculate　the　flexural　stiffness　of　composite　slim　beams　with　deep　deck　is　proposed　by　the　method　of　equivalent　cross-section　as　well　on　the　principle　of　equivalent　stiffness　and　the　feasibility　of　the　formula　is　testified　by　the　experimental　results.　The　error　between　the　calculation　results　and　test　results　is　less　than　6.0%　when　the　deflection　at　mid-span　reaches　1/400　of　the　beam　span,　and　turns　to　be　less　than　3.0%　when　the　deflection　at　mid　span　reaches　1/250　of　the　beam　span　and　the　calculation　results　is　conservative.　The　flexural　behavior　of　composite　slim　beams　and　the　strain　distribution　of　sections　have　been　investigated　by　experimental　test.　The　results　indicate　that　the　deformation　of　the　beam　section　remains　plan　during　the　elastic　stage.