5.9%。Based on the water vapor balance in greenhouse, a dynamic forecasting model of humidity directly as a function of the inside and outside climate, greenhouse structure, crop growth, and soil moisture was presented. Meanwhile, some physical processes correlative with the water vapor change such as crop transpiration, soil evaporation, covering material condensation, and natural and mechanical ventilation were quantitative analyzed. To validate the correctness of the model, an experiment was carried out in the plastic multi-span greenhouse in North China during winter. The inner relative humidity forecasted agreed well with the measured data, with the root mean square error of 5.9%. 