This paper examines the symbiotic relationship between low-carbon growth and the port economy in coastal towns, utilizing composite system theory and focusing on Lianyungang, China, as a case study. This study analyzes the evolutionary aspects of the connection from 2010 to 2019 by developing static and dynamic symbiotic evaluation models. The data indicate a sequential transition: starting with disorder (2010–2012), the system advanced through phases of slight imbalance (2013–2015) and moderate coordination (2016–2017), ultimately reaching high-quality coordination (2018–2019). Key drivers included policy interventions and industrial restructuring, though the port economy’s reliance on traditional bulk cargo transport initially constrained low-carbon potential. However, obstacles remain, such as infrastructural deficiencies and the protracted development of low-carbon industries. This research provides a quantitative methodology for evaluating port-city sustainability in the context of transportation and energy integration, offering policy insights for achieving carbon neutrality while ensuring economic growth. Future research could enhance forecast precision by incorporating nonlinear dynamics.