Abstract: A dynamic greenhouse thermal and humidity mechanism model for multi-span greenhouse with multi-layer covers was developed in this paper. The model describes the physical processes such as convection, conduction in soil, solar radiation, long-wave thermal radiation, transpiration, evaporation and condensation on covering materials, natural and mechanical ventilation that govern the thermal environment of greenhouses, and establishes mass and energy equilibrium equations of covering materials, indoor air, plant and soil. By resolving these equations, this dynamic model can be used to predict the indoor temperature, humidity, energy and steam flux density among greenhouse components, such as covering materials, soil, plant and so on, when boundary condition is determined. A procedure that is able to calculate radiation heat exchange among internal and external curtains, internal and external covering materials, plant canopy, ground surface and sky is deduced based on the concept of net radiation. Using pressure distribution method, a formula is put forward to calculate the ventilation of greenhouse equipped with multi-ventilation inlets such as roof windows, side windows and wet-pads under wind pressure and thermal pressure. Comparing with previous models developed by other researchers, the model presented in this paper has wider range of application.