The multi-fluid model (MFM) has been widely used in the simulation of gas-solid fluidized bed reactor, in which the density and diameter of particles normally change due to the heterogeneous reactions. However, the particle diameter always keeps constant although the particle density varies along with reaction in the previous MFM, which may disobey the law of mass conservation at the particle scale. Therefore, in order to overcome the limitation of constant diameter in the previous MFM, the biomass pyrolysis reaction was taken as an example to develop the variable particle density and diameter model based on the mass conservation at the particle scale, which was successfully incorporated into MFM to simulate the biomass pyrolysis process in a fluidized bed reactor. The results indicate that the segregation/mixing and entrainment behavior are significantly affected by the variation model of particle density and diameter and the spedes density in the solid phase. (C) 2016 Elsevier B.V. All rights reserved.