Supercritical water fluidized bed (SCWFB) is a newly developed reactor for coal and biomass gasification without releasing pollutants. The knowledge of two-phase flow properties in SCWFB is still lacked so far in literatures due to the extreme operating condition of high-pressure and high-temperature supercritical water. In this paper, a special dual-capacitance probe measurement system is developed for the extreme flow environment in SCWFB. It is found that the linear relationship is more precise in converting the capacitance of the probe sensor into local volume fraction of solids in SCWFB than Bottcher relationship. With the help of the newly-developed dual-capacitance probe measurement system, an experimental study on the bubbling fluidization in a SCWFB is carried out when the system pressure is in the range of 20-27 MPa and the system temperature is in the range of 410-570 degrees C. Four groups of quartz sands with different mean diameters which belong to Geldart type B classification are fluidized. It is found that the bed expansion method which was used in literatures to identify the onset of bubbling fluidization in supercritical carbon dioxide fluidized bed doesn't have enough precision in a SCWFB. To solve this problem, a new method of identifying the onset of bubbling fluidization in SCWFB is proposed. The minimum bubbling Reynolds number increases with the Archimedes number in a logarithmic relationship in SCWFB. At last, a new predicting correlation of the minimum bubbling fluidization velocity in SCWFB is proposed: Re-mb = 26.7091n(Ar + 10253.88) - 247.339, 1900 < Ar < 180000 The relative error of the new predicting correlation is within +/- 20% and the averaged relative error is 7.5%. The research work in this paper provides useful guidance for the optimization and operation of SCWFB reactor. (C) 2019 Elsevier Ltd. All rights reserved.