Abstract: A comparative study of flow characteristics in three-dimensional supercritical water fluidized beds (SCWFB, 440–560 ℃, 22–26 MPa), cold gas-solid fluidized beds (CGSFB, 20 ℃, 0.1 MPa), and high-pressure and high-temperature gas-solid fluidized beds (HGSFB, 440–560 ℃, 22–26 MPa) was conducted via two-fluid model simulations across different fluid velocities, un-fluidized bed heights, pressures and temperatures. Bubble (or slug) and particle dynamics, fluid velocities, and bed expansions are investigated. The results show that, the drag forces of particulate phase are around 0.4 times particulate phase weights in SCWFB. Compared with CGSFB, SCWFB has lower minimum fluidization velocities, smaller bubble and slug sizes, more bubble and slug numbers, higher bed expansions and slug onset heights, and more uniform gas-solid interactions. Flow structures in SCWFB are similar to those in HGSFB. Pressure and temperature have less effects on the flow structures than fluid velocity and un-fluidized bed height in both SCWFB and HGSFB.