Natural gas is a kind of clean, efficient green energy source, which is used widely. Liquefied natural gas (LNG) is produced by cooling natural gas to -161 degrees C, at which it becomes the liquid. Once LNG was released, fire or explosion would happen when ignition source existed nearby. The high expansion foam (Hi-Ex foam) is believed to quickly blanket on the top of LNG spillage pool and warm the LNG vapor to lower the vapor cloud density at the ground level and raising vapor buoyancy. To identify the physical structure after it contacted with LN2 and to develop heat transfer model, the small-scale field test with liquid nitrogen (LN2) was designed. In experiment, three layers including frozen ice layer, frozen Hi-Ex layer and soft layer of Hi-Ex foam were observed at the steady state. By characterizing physical structure of the foam, formulas for calculating the surface of single foam bubble and counting foam film thickness were deduced. The micro heat transfer and evaporation model between cryogenic liquid and Hi-Ex foam was established. Indicating the physical structure of the frozen ice layer, there were a certain number of icicles below it. The heat transfer and evaporation mathematical model between the frozen ice layer and LNG was derived. Combining models above with the heat transfer between LNG, ground and cofferdam, the heat transfer and evaporation mathematical model of LNG covered by Hi-Ex foam was developed eventually. Finally, LN2 evaporation rate calculated by this model was compared with the measured evaporation rate. The calculated results are 1.2-2.1 times of experimental results, which were acceptable in engineering and proved the model was reliable. (C) 2016 Elsevier Ltd. All rights reserved.