Eco-industrial parks (EIPs) promote the coordination of economic development and environmental protection. This paper uses the nonlinear DID method, with the data of 288 cities spanning from 2003–2019, to study the nonlinear effects of EIPs on SO${}_2$ and CO${}_2$ emissions, aiming to portray the nonlinear and heterogeneous characteristics of EIP’s effects. Meanwhile, this paper examines the effects of EIPs more accurately and completely. The main results are as follows: 1. EIPs can significantly reduce CO${}_2$ and SO${}_2$ emissions, but there is significant heterogeneity between the effects. 2. The effect of EIPs on SO${}_2$ and CO${}_2$ emissions is nonlinear. In addition, it shows significant nonlinear characteristics as the change of foreign investment and population density in cities. Therefore, it is important to consider these nonlinear characteristics when establishing and evaluating EIPs. This paper accurately identifies the nonlinear effects of EIPs and provides some suggestions for the future development of EIPs.