Abstract: Geothermal greenhouse is the main content of geothermal applications in agriculture. The calculation of heating load in geothermal greenhouse is the main foundation of designing geothermal greenhouse, it affects normal growth of greenhouse vegetable and utilization scale of geothermal energy.According to the ecological demand of greenhouse vegetable, the local meteorological condition and the construction of outer enclosure, the heating load of geothermal cgreenhouse is calculated by heat transfer, can't be determined only by reference to the data of other places.The reasonable selection of calculating temperature of greenhouse outside is the key of calculating heating load. This paper puts forward a new method of assurance coefficient to determine the calculating temperature of greenhouse outside, it can expand the utilization scale of geothermal energy as well as assure the normal growth of greenhouse vegetable.In the paper, following the example of geothermal greenhouse in Xiong County, the calculatiog method of heating load in greenhouse is set out. The comparative analyses of calculating results versus measuring results show that the method used in the paper is feasible.