Abstract: Based on the principle of "Black—box" , this paper analysed the input and output parameters of honeycomb paper pad system used for lowering the temperature in greenhouses.The relationships between these input and output parameters are complex because they are mainly affected by both the structural geometric sizes of the pad and the air flow velocity passing through the pad. The orthogonal experimental method is used to analyse the pad system.According to the orthogonal experiments and some other supplementary experiments, a set of empirical formulae for evaporative efficiency and state pressure difference were derived with the least square method. By these formulae, an optimal set of the structural geometric sizes is calculated.The calculated result shows that the evaporative efficiency is higher than 80%. As the author of this paper made several peformance tests of the pad with the calculated structural geometric sizes the results were satisfied.