As the country with the world"s largest carbon emissions, China is developing a green and low-carbon economy and taking a good road of sustainable development. Based on the inter-provincial panel data of 2004-2017, based on the external effect of industrial agglomeration, the mediation effect analysis method was used to study the impact of industrial agglomeration on carbon emissions, and foreign direct investment was used as a threshold variable to further analyze the relationship between the two. The following conclusions are drawn: (1) Foreign direct investment plays a partial mediating role in the process of industrial agglomeration affecting carbon emissions, with a mediating effect value of 31.1%. (2) There is a double threshold effect between industrial agglomeration and carbon emissions, and the relationship between the two is inverted U-shaped. When foreign direct investment is at a low level, industrial gatherings increase carbon emissions. When foreign direct investment levels cross a higher threshold, the positive externalities of industrial agglomeration are highlighted, and carbon emissions are suppressed. (3) The level of economic development and energy structure have a significant positive impact on carbon emissions, and industrial structure optimization can inhibit the increase of carbon emissions.(4) According to the Kuznets curve test, the relationship between industrial agglomeration and carbon emissions is an inverted U-shape, and the critical value of industrial agglomeration is 2.389.