The dual fluidized bed (DFB) reactor is promising to convert biomass into high-quality syngas efficiently. In this work, a three-dimensional model is built based on the granular kinetic theory to predict the biomass steam gasification in dual fluidized bed reactors. The model is firstly validated against a series of experimental results. Then, the effects of some essential operation parameters including the biomass flow rate (F-b), the steam to fuel ratio (R-sf) and the gasification temperature (T-g) on the biomass steam gasification properties in a DFB reactor are comprehensively analyzed with the orthogonal method. In the concerned ranges of the operation parameters, the cold gas efficiency is found to be the most sensitive to F-b and least sensitive to T-g. The optimal cold gas efficiency of the DFB gasifier is 82.9% when F-b, R-sf and T-g are 15 kg/h, 1.5 and 900 degrees C, respectively, and the H-2 mole fraction is 46.62%.