In the context of rapid urbanization and demographic transition, the implications of population shrinkage for urban sustainable development have attracted increasing scholarly attention. Nevertheless, empirical evidence on the relationship between population change and urban ecological resilience remains limited. Drawing on the Pressure–State–Response (PSR) framework, this study constructs a comprehensive indicator system to assess urban ecological resilience in 110 cities along the Yangtze River Economic Belt (YEB) over the period of 2012–2021. Furthermore, a panel threshold regression model is employed to examine the nonlinear effects of population shrinkage on urban ecological resilience. The findings indicate that urban ecological resilience exhibits an overall upward trend in YEB, characterized by pronounced spatial disparities. Eastern cities have a higher level of resilience than cities in the western region in YEB. The number of cities with shrinking populations is gradually increasing, and these shrinking cities are mainly small and medium-sized cities. The empirical results show that the impact of population shrinkage on urban ecological resilience is distinctly nonlinear, and regional economic development plays a moderating role in this nonlinear relationship. At lower levels of economic development, population shrinkage does not significantly moderate urban ecological resilience. As the economy reaches a moderate stage, population shrinkage exerts a stronger modulatory effect on ecological resilience. When economic development advances to a higher level, however, population shrinkage tends to inhibit ecological resilience. Overall, this study provides a scientific basis for the population–ecological policies tailored to local conditions and offers valuable insights to promote urban sustainable development under conditions of population shrinkage.