Since the establishment of China's Pilot Free Trade Zone, it has faced certain environmental pollution problems while expanding trade openness. Based on the panel data of 180 cities from 2007 to 2017, this paper uses the multi time point double difference method (DID) to test the environmental effects of the establishment of free trade zones in each city. The empirical results show that: (1)The construction of the free trade zone has played a significant role in carbon emission reduction, which can reduce the growth rate of carbon emissions by 2.0% compared with that before the establishment of the free trade zone, and this effect is still valid under a series of robustness tests such as superimposed policy review, counterfactual test, excluding the impact of the financial crisis, PSM-DID, etc.; (2) Through the intermediary effect, it is found that the establishment of the free trade zone has effectively mitigated the growth of carbon emissions through the combined effects of scale effect, structure upgrading effect, resource allocation effect and green technology innovation effect, and the four effects have a positive impact on the environment; (3) Based on the regional heterogeneity, this paper finds that the establishment of pilot free trade zones in coastal areas, especially in the Pearl River Delta, can significantly slow down the growth of carbon emissions, while the establishment of free trade zones in the Yangtze River Delta has also slowed down the growth of carbon emissions, but failed to pass the significance test. For inland areas, the inhibition of the establishment of free trade zones on the growth rate of carbon emissions is not obvious; (4) Cities with a permanent population of 5million can significantly inhibit carbon emissions, but it is not significant for cities with a permanent population of less than 5million.