Abstract: Based on the measured net radiation data above the canopy and on the soil surface under the crop canopy in winter wheat and summer corn field,a calculation model of the ratio between crop leaf transpiration and soil evaporation from farmland has been established in this paper.The model can express the physical oaring laws of the ratio correctly and is practical and useful.The research result shows that the ratio between leaf transpiration and soil evaporation a is increased with the leaf area index increasing,has a daily change and is higher in mid-noon and lower in the early morning and near night.The daily change process is influenced by leaf stomata movement.