Measuring County-Level Coordination of Urban Construction Land and Population in China: A Coupling Coordination Degree Approach Across 2838 Counties

China’s county towns have become a key carrier of new-type urbanization, yet the coordination between urban construction land (UCL) and urban population (UP) remains underexplored at the county level, particularly because prior work conflates urban and rural areas and assumes linear, globally uniform drivers. Using official 2024 data for 2838 Chinese counties, this study develops a county-level coupling coordination degree (CCD) between UCL and UP and refines it with a per capita urban construction land (PCL) benchmark (115–150 m2/person, GB 50137-2011). Spatial analysis shows a pronounced east-high-west-low gradient of CCD, with 48.0% of counties classified as imbalanced; a Theil decomposition attributes 58.7% of CCD inequality to within-province differences, indicating that the most consequential coordination gaps operate below the provincial scale. An interpretable XGBoost-SHAP model identifies driver importance, nonlinear thresholds, and coordination-class-specific weight heterogeneity. Results show that tertiary industry output and GDP dominate coordination everywhere, while location constraints fade and fiscal–public-service levers rise with coordination; TIV, GDP, and high-school provision exhibit clear saturation or threshold effects. By restricting the analysis to urban areas and exposing nonlinear, class-specific drivers, the study provides a differentiated, evidence-based basis for coordination-oriented land and population governance.