The　robust　control　based　on　a　velocity　observer　was　introduced　for　a　free-floating　flexible　space　manipulator　with　unknown　payload　parameters　and　bounded　disturbances.　The　dynamic　model　of　the　free-floating　space　manipulator　with　a　flexible　link　was　established　by　applying　the　momentum　conservation　principle　and　the　Lagrange　equations.　The　singular　perturbation　model　of　the　flexible　space　manipulator　system　was　obtained　according　to　the　two-time　scale　control　theory,　in　which　the　system　was　decoupled　into　slow　(rigid)　and　fast　(flexible)　subsystems.　Then　a　composite　controller　consisting　of　a　slow　control　component　and　a　fast　control　component　was　proposed.　A　sliding　observer　based　robust　control　algorithm　was　applied　to　control　the　slow　subsystem　with　unknown　payload　parameters　and　bounded　disturbances　to　track　the　desired　trajectory.　The　fast　controller　was　designed　with　the　estimated　velocity　based　on　a　linear　observer　to　damp　out　the　vibration　of　the　flexible　link　using　optimal　linear　quadratic　regulator　(LQR)　method.　Finally,　a　numerical　simulation　was　carried　out,　which　confirms　the　controller　proposed　was　feasible　and　effective.　The　virtue　of　this　control　scheme　is　that　the　linear　position,　linear　velocity,　linear　acceleration　and　angular　velocity　of　the　base,　the　angular　velocities,　angular　accelerations　of　the　joint　as　well　as　the　derivative　of　the　flexible　vibration　mode　needn＇t　be　measured　directly.　©,　2015,　Chinese　Vibration　Engineering　Society.　All　right　reserved.