This study investigates the causal interactions during heatwaves across 520 urban sites in the U.S. using complex network and causal analysis. The results show that megacities, such as New York City and Chicago, are causally connected with most other cities and mediate the structure of urban networks during heatwaves. A significant positive correlation is identified between the causality strength and the total populations in megacities, indicating the impact of human activities on urban heatwaves. The study also reveals the emergence of teleconnections and supernodes, which are informative for predicting and adapting to heatwaves under global climate change. The findings highlight the critical role of megacities in the propagation of extreme heatwaves and provide insights into urban sustainability and climate resilience.This study investigates the causal interactions during heatwaves across 520 urban sites in the U.S. using complex network and causal analysis. The results show that megacities, such as New York City and Chicago, are causally connected with most other cities and mediate the structure of urban networks during heatwaves. A significant positive correlation is identified between the causality strength and the total populations in megacities, indicating the impact of human activities on urban heatwaves. The study also reveals the emergence of teleconnections and supernodes, which are informative for predicting and adapting to heatwaves under global climate change. The findings highlight the critical role of megacities in the propagation of extreme heatwaves and provide insights into urban sustainability and climate resilience.