To assess the impact of offshore wind farms (OWFs) on the structure and energy flow of coastal ecosystems, Ecopath models of the Jiangsu coastal ecosystem (JCE) based on biological field data collected before and after the establishment of the Rudong OWFs in 2007 and 2015, respectively, were constructed and compared. The results indicated that after OWF construction, detritus, phytoplankton, zooplankton, anchovies, and some benthic fish were positively impacted. The increased primary production and detritus resulted in the increased food supply for zooplankton, which made it possible for planktivorous species (particularly anchovies) to be fed. Consequently, the biomass and production of some benthic fish increased, which indicates a potential reef effect. Other groups with decreased biomasses and productions may have been negatively impacted by the OWFs. Herbivory flows dominated the pre- and post-construction JCE despite their low transfer efficiencies; however, the proportion of detritivory flow increased after OWF construction, and this was especially prominent at high trophic levels. The post-construction JCE was immature with lower system connectivity, trophic flow utilisation, and transfer efficiency. However, the ecosystem tended to develop towards higher maturity with higher energy throughput, ecosystem activity, and recycling capability.