Supercritical water (SCW) fluidized bed reactors convert biomass to fuels without pollutants emission.In this work,experimental studies were carried out to investigate voidage distribution in an SCW flu-idized bed by capacitance probes.Quartz sands with different particle sizes were fluidized by SCW under system pressure of 20-27 MPa and temperature of 410-570 ℃.The effect of operation conditions on voidage distributions of the emulsion phase (e.g.averaged voidage and probability density) is discussed.A predicting correlation between voidage and superficial velocity in emulsion phase is proposed.The relative error of the correlation is within ±25％.These research results provide useful guidance for the optimization of supercritical water gasification technology.