Considering carbon partial conversion and heat loss of the system, a thermodynamic model of biomass gasification was established based on mass balance, energy balance and chemical equilibrium, which was solved with Newton-Raphson method. The model is validated by comparison with the literature data. The effects of air preheat temperature, moisture content, reaction temperature and equivalence ratio on gasification performance were analyzed by the model. The detailed process of modeling and the calculating results are valuable for the development of the biomass gasification.