In order to obtain the threshold value of carbon emission in urban green development of Hunan Province, an international threshold regression method has thus been adopted. Based on the impact of urban development efficiency on carbon emissions, a selection has been made of 14 basic variables including urban population, per capita income in urban and rural areas, gross import and export value, which reflect the degree of urban openness, as well as their corresponding data from 2008 to 2015. The determination of the threshold value of interaction between variables on carbon emissions can be achieved by the regression analysis of panel threshold or double panel threshold. The results show that with the urban population density reaching 7 818 capita/km2, the urban area expansion and carbon emissions show a reverse change. When the per capita disposable income of urban residents reaches 21 018 yuan per year, the increase of urban population leads to a reduction of the carbon emission. With the industrial gross value less than 0.45 of the total GDP, the increase of import and export of foreign trade helps to reduce carbon emissions. With more than 6 673 technology mobile personnel in enterprises per year, the increase in urban population helps to reduce carbon emissions. With foreign investment enterprises exceeding 93, FDI has a significant effect on the reduction of carbon emissions. FDI accounts for more than 30.39% of total GDP, and the increase proportion of FDI of total GDP helps to reduce carbon emissions. In addition, the increase of urban population has no obvious impact on carbon emissions with an urban green area larger than 4.01 km2 or that accounting for 3.35% of urban area.