o explore the microclimate factors, such as air flow, temperature, humidity and CO2 concentration inside a single span plastic greenhouse, an environmental model was established including components of greenhouse such as inner and outer space, plant and soil layer, etc. The air was assumed to be composed of dry gas, water vapor and CO2. Based on the solar radiation, mass-thermal exchange of plant-environment, transmitting process of energy and momentum of various parts, three dimensional simulation was executed to predict the pattern of temperature, humidity and CO2 concentration. The Monte Carlo method was adopted in the model of thermal-radiation transport process. Plant was simplified as continuous solid medium, and shear stress transport (SST) model was used in air turbulence analysis. The results showed that the ventilation pattern had an obvious effect on distribution of temperature, humidity and CO2 concentration. The values of temperature, humidity and CO2 concentration in upwind area were low, and the environment in downwind area was not favorable for crop growing. Generally, the predicted values were a little lower than those of the measured, while the trend was similar. The relative error values of temperature, humidity and CO2 concentration were less than 8%, 6% and 7%, respectively.