Abstract: In this paper, a model is presented to describe pyrolysis of biomass in a fluidized bed reactor, which is characterized with reaction routes related to the biomass particles either in dense phase or in dilute phase.Model calculation indicated that the residence time of particles differed greatly from that of gas product, so product distribution from pyrolysis was significantly affected.Agreement between calculation and experimental data justified the model’s predictability in pyrolysis reaction of biomass carried out in the fluidized bed reactor.Based on the calculation, factors including reaction temperature, flow rate of fludizing gas, feed rate of biomass and particle size,were analysed in order to estimate their effects on the results expected from pyrolysis.