ABSTRACT Whether local governments effectively implement environmental policies is crucial for successfully addressing global environmental challenges. However, scholars are still unclear about when and why local implementation of environmental policies fails, especially in developing countries where high priority is often given to economic development and where local officials are held accountable upwards. In this study, we investigate how prefectural city governments implement environmental performance assessment policy (EPAP) in China. Based on a sample of prefecture-level cities from 2005 to 2015 and a difference-in-differences estimator, we find that the EPAP has effectively reduced pollution emissions. However, the pollution abatement effect only emerges in assessment years but not in non-assessment years, indicating an opportunistic implementation of environmental policy at the city level. The opportunistic implementation behaviour is more evident in cities where emission reduction task is higher and where city mayors lack environmental work experience or professional expertise.