The fast pyrolysis of biomass in a bubbling fluidized-bed reactor was simulated with the multi-fluid model combing the variable particle density and diameter model based on the mass conservation at the particle scale. Different particle shrinkage effects on the reactor performance were investigated through changing the apparent density of char species. The detailed distributions of particle density and diameter in the reactor and entrained out of the system were revealed. The results demonstrate that the reactor performance, including the particle density and diameter distribution, entrainment behavior, biochar composition and yield, and biomass conversion, is dramatically affected by the particle shrinkage effects. The average particle density increases, while the average particle diameter decreases, with the increase of the char density, which means more intense shrinkage. A weaker shrinkage effect leads to stronger entrainment behavior, larger biochar yield and mass fraction of biomass in biochar, and lower biomass conversion.