Promoting city lowcarbon construction is the important measure for realizing the highquality development of carbon peak and neutrality. This essay is based on the annual data of Shenzhen and Shanghai A Share industrial enterprises, taking such empirical methods as three times lowcarbon trial policy as quasinatural experiment, adopting multiperiod difference in difference model, recursion medium effective model to investigate the mechanism for reduction of carbon emission and the interrelationship in the course of environmental audit. The examination result demonstrates that 1) the policy of lowcarbon trial city remarkably reduces the emission volume of trial city enterprise, what is more, the emission effect manifests itself in the highcarbon enterprises, and enterprises in the highlydeveloped industrial areas, and the enterprises worthy of media attention; 2) the complementary relationship between the policy of lowcarbon trial city and the effect of environmental audit in promoting enterprise reduction of carbon emission; 3) the mechanism examination shows that the innovation of enterprises plays a partial medium roles in the reduction of carbon emission of lowcarbon city trial policy.