Waterfront buildings are increasingly popular in river-dense cities such as Changsha, where water bodies contribute significantly to urban microclimate regulation. However, uncoordinated building spatial design can compromise these benefits and lead to suboptimal outdoor thermal comfort. Existing studies often focus on isolated factors without addressing their combined effects on outdoor thermal comfort. Following ENVI-met simulation, Morris sensitivity analysis, and Generalized Additive Modelling, this study identifies dominant morphological factors and assesses their seasonal synergistic impact. Building upon these results, a comprehensive thermal comfort index tailored to waterfront environments (PETCIW) is proposed, integrating spatial, environmental, and seasonal variables through the Analytic Hierarchy Process and multiple linear regression analysis. Based on 40 residential cases, the study develops climate-responsive design strategies that balance seasonal needs. The findings offer a quantitative foundation and practical guidance for early-stage urban design in waterfront areas, contributing to more resilient and thermally adaptive urban planning.