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The supercritical water fluidized bed (SCWFB) is a new concept of reactor for coal and biomass gasification without releasing pollutants. In this paper, a special dualcapacitance probe measurement system is developed and a comprehensive study on bubble hydrodynamics in SCWFB is carried out. Four groups of particles with different mean diameters were fluidized by supercritical water with the pressure ranges from 20 to 27 MPa and temperature ranges from 410 to 570 °C. The effect of operating parameters on bubble size, frequency, and rising velocity is discussed in detail. Special rules of bubble hydrodynamics are observed, and the theoretical mechanisms are revealed. New predicting correlations of bubble diameter and bubble rising velocity are proposed. The results in this paper expand the research of fluidization under extreme operating conditions and also provide useful guidance for the optimization of the reactor.