Abstract: Supercritical water fluidized bed (SCWFB) reactors are designed to gasify biomass or coal with high efficiency. In this paper, the wall-to-bed heat transfer characteristics in SCWFB are studied using the computational fluid dynamics and discrete element method (CFD-DEM) coupled with a constant heat flux boundary. Two different methods are considered to deal with the multiphase heat flux boundary in CFD-DEM because there is currently no single widely accepted approach. Zhang’s method predicts a more accurate wall-to-bed heat transfer coefficient in SCWFB than Lattanzi’s method according to comparisons of the simulation results with an empirical correlation. The influences of temperature, pressure, velocity, and the solid phase properties, such as the particle diameter, particle heat capacity, particle thermal conductivity, and particle density, on the SCWFB wall-to-bed heat transfer characteristics are studied based on Zhang’s method. The simulation results help reveal the SCWFB heat transfer characteristics.