What determines the synergy among urban systems? Evidences from the Yangtze River economic belt over 20 years

China's urbanization has reshaped landscapes, economies, and societies nationwide at unprecedented paces, yet inconsistent march among these processes has resulted in insufficient and unbalanced urban development. Here, we constructed a Population-Land-Economic-Social urbanization (PLESU) system with multiple elements and interaction flows, and explored the spatiotemporal dynamics and influencing factors of the coupling coordination degree (CCD) among PLESU system in 110 prefectural-level cities of Yangtze River Economic Belt (YREB) from 2000 to 2020. Results indicated that the CCD evolution exhibited significant path dependence during the past two decades, with considerable room for improvement. A key finding was that the low-quality development of land urbanization and social urbanization subsystems was the main factor for the lack of synergy among PLESU system, with land finance dependence and municipal investment confirmed to have significant impact on this. Furthermore, results also highlighted narrowing inter-regional differences in the CCD among downstream, midstream and upstream, revealing clustering of CCD in urban agglomerations, yet differing in their internal differentiation patterns and drivers. Exploring the synergy among urban systems will raise policymakers’ awareness of the necessity for embracing a holistic approach toward better sustainability by considering the insufficient and unbalanced nature of urbanization.