Abstract: Abstract: The quantity of nocturnal heat loss from conventional single skin greenhouse is high due to low thermal resistance of the single plastic materials. Therefore it is necessary for solar greenhouse that the thick covering material is covered on the single plastic covering material at night in China. There are lots of studies on heat transfer through thin cover of greenhouse, but limited reports about heat transfer through thick cover. However, the characteristics of heat transfer through thick covering materials of solar greenhouse are different from through thin covering. In order to master the heat transfer properties and thermal insulation characteristics of thick covering materials and its influencing factors, a model of heat transfer through greenhouse thick covering materials was developed based on the heat transfer theory in this paper. According to the theoretical model, a computer program was also designed and compiled to simulate the radiation, convection, conduction and other forms of heat transfer process of thick covering materials, and the quantity of heat transfer and the heat transfer coefficient were calculated. In the model there were some assumptions: the heat transfer process was regarded as a steady-state heat transfer; the sky was regarded as an infinite blackbody; the plant, ground, wall, roof inside the greenhouse were considered as one radiator which participated in radiation heat transfer with the under surface of thick cover, and the temperature, radiation emissivity and reflectivity of the radiator were the averages of theirs; the ground and the sky outside the greenhouse were also considered one radiator that participated in radiation heat transfer with the upper surface of thick cover; the surface of the thick cover, indoor ground, plant, wall, roof were regarded as grey body; the condensation heat transfer on the surface of the cover was neglected. The model used an analysis method on the effective radiation and the projection radiation of each surface to determine the quantity of radiation heat transfer between the upper surface and outdoor environment, the under surface and indoor environment. According to the principle of energy balance, the relationship between each part with various forms of heat transfer of thick cover was established. Based on the thermophysical characteristics, the construct parameters of greenhouse and the environment conditions etc., the heat transfer flux density and heat transfer coefficient of covering could be simulated and forecasted by this model. There was an apparatus to measure the overall transfer coefficient of covering materials, which was a static-hot box device. We used the device to test seven different combination types of thick covers of insulation materials. The results showed that the simulated heat transfer coefficient of thick covers was consistent with the values from experiments. This indicated that the theoretical model is highly accurate. The model can be used for quantitative analysis of heat transfer through thick covers and evaluation on the thermal insulation property. The theoretical model provides a method for the thick cover research and development as well as analysis of thermal environment about horticulture.