The　supercritical　water　fluidized　bed　(SCWFB)　is　a　recently　introduced　reactor　for　biomass　gasification　that　does　not　release　pollutants.　Four　groups　of　Geldart　B-type　quartz　sands　with　different　particle　sizes　were　fluidized　at　a　system　pressure　of　20–27　MPa　and　system　temperature　of　410–570　°C.　A　series　of　experiments　were　performed　for　determining　the　drag　coefficient　and　volume　fraction　of　bubbles.　The　effects　of　the　particles’　size,　superficial　velocity,　system　pressure,　and　temperature　on　the　drag　coefficient　and　volume　fraction　are　discussed.　In　addition,　a　correlation　between　experimental　and　computed　values　is　demonstrated　for　both　the　drag　coefficient　and　volume　fraction　in　SCWFBs.　The　relative　error　of　the　correlation　is　within　±30%.　The　results　of　this　study　provide　significant　guidance　for　the　scaling-up　design　of　SCWFBs　and　for　the　development　of　supercritical　water　gasification　technology.　©　2021　Chinese　Society　of　Particuology　and　Institute　of　Process　Engineering,　Chinese　Academy　of　Sciences